2 months in Europe & 2 cruises. Packing advice needed

Well, there are details we don't have...like whether you prefer to dress for formal nights on cruises. Or whether your cruise wardrobe and land wardrobe are essentially the same.
A LBD with a variety of accessories can cover your more fancy nights.
Laundry is your best friend. Either accept doing your own sink laundry, using ship laundry, or planning your outfits to last between land laundry.
I regularly travel 4- 6 weeks in an expanding carry on size rolling suitcase and a purse. I do have a rolling tote bag for flights to accommodate the non expanded suitcase requirements. I do a lot of sink washing, but as I only have one my own clothes to deal with, it works.
I accept that my shoes won't be the pretti3st, but as I need orthotics and flat tie ups, I accepted that long ago. So 2 pair of walking shoes, flip flops for slippers and pool use, and a lightweight, compact able dress shoe when necessary.
A merino wool cardigan is very adaptable for weather changes and something I always carry.
There are dozens of travel capsule wardrobe YouTube videos for carry on only including cruises.
Reality is, what you pack for a week can easily be stretched to many weeks' travel.

Will you have easy laundry access in your accommodations? You really only need what you need between laundry days.
Where in Spain--you may need vastly different wardrobe from UK, or not.
I see the benefit of this when flying, but in your case, I'm not so sure...

You are not nuts. What you propose is doable. We go on three week cruises, and trips to Europe with only a carry on and personal bag apiece. Combining the two types of trips might require dressing down some of the cruise wear and dressing up some of the Europe wear. Shouldn't be a problem, as most cruise lines now have a more "smart casual", less formal dress requirement. And nothing wrong with looking smart while in Europe.

General thoughts:

Take only black, white, and grey clothing that can be mixed and matched. This works for men as well as women. Women can add a splash of color with a scarf or tee or tank. Men can add color with shirts. Take things that are light weight, wrinkle free, and will dry quickly. Travel Smith, and Chico's are good sources. Both have web sites and Chico's has regular stores.

Take very little jewelry. One or two necklaces and ear pieces at the most. Take only two purses at the most. Your regular day bag will do ok for daytime. If you feel the need for another then a small clutch for evening.

Before I forget it, no matter if you only use a carryon, always put half of your stuff is his bag and half of his stuff in your bag. Carryon bags are sometimes gate checked no matter what the size / weight. If one of the bags goes on its own vacation, you will still be able to get by.

Clothing for Her:
Three under shirts – two regular and one tank top – wear one and pack two.
Five underwear -- wear one and pack four.
Three bras -- wear one and pack two.
Five pairs of socks – wear one and pack four.
Two pairs of shoes – wear one and pack one.
Three pairs of pants – wear one and pack two.
Three shirts or blouses – wear one and pack two
One jacket and one belt – both to be worn in route.
One vest, one sun hat, and one light sweater – all to be packed.
Rain coat to be packed.

Clothing for Him:
Three under shirts –wear one and pack two.
Five underwear -- wear one and pack four.
Three handkerchiefs – have one in pocket and pack two.
Five pairs of socks – wear one and pack four.
Two pairs of shoes – wear one and pack one.
Three pairs of pants – wear one and pack two.
Three shirts – wear one and pack two
One jacket and one belt – both to be worn in route.
One vest, one sun hat, and one light sweater – all to be packed.
Rain coat to be packed.

This is the bare bones packing list. You might be able to add another shirt or pair of pants for each – just depends on how much space is taken up with cosmetics, shaving gear, phone chargers, guide books, and misc.

Each of us also carries a RS Veloce Shoulder Bag as a personal item. It will hold lots of odds and ends.
This list isn’t perfect for everyone but it works well for us. At least it will serve as a starting point for developing your own.

We also use a combination of packing cubes, folders, and tubes to organize things and aid in the placement of the various items. One of the keys to being able to travel for a long period with only a carryon is experimenting with the packing well before you get ready to go.

Three things to consider. First, laundry will have to be done along the way. We do all ours by hand and it only takes 20 or so minutes a day. Second, you will wear the same outfit more than once on the cruise. Maybe two or three times each. Third, do not worry about what others one the cruise may think if they see you in the same outfit several times. They don't pay your bills.

Hope this helps you get started.

My husband and I have twice done a transatlantic cruise followed by a month in Europe, with only carry-on luggage, so this is doable. The most important factor is weight. Weigh everything. You’ll be surprised how much difference there can be between similar-looking items. I have substituted Athleta Brooklyn pants for Chico’s and saved a lot of weight.

I agree to mainly focus on black and white, but I like to add a color — maybe a very lightweight dressy top for special dinners.

Your fellow cruisers will not even notice your repetitive wardrobe.

The one tip my friend gave when returning from her cruise was to buy the laundry package for the boat, if one is offered. She sent items to be cleaned every day. Said she met many people onboard her ship who go on month long cruises with just a carry-on because they wore something, sent it to be cleaned, and it was returned the next day.

Weighing your clothes will allow you to make better choices. You can write the weight of each garment on the tag using a fine-point sharpie, but I find it's easier to make a list of the clothing I am thinking about taking, grouped by type, then note the weight next to the item. That makes it easy to choose when you can see the weights side-by-side.
It's been a long time since I was on a cruise, but it's been my understanding for awhile that formal nights are not as strict as they used to be, and there was always another option available for those who didn't want to bring (or rent--is that still a thing?) formalwear.
A garment that I recommend for this type of trip is a sleeveless no-iron cotton button-up shirt. I had a white one that I traveled with pretty extensively, despite it weighing more than some of my other tops. That stiff collar really looks and feels like "real" clothes, not the borderline hiking clothes I normally tromp around in.
I agree with the comment about getting the laundry service on the ship. I would bring a zippered mesh bag for socks and underwear so they don't get lost. If you've been traveling with the same cruise line all this time, you may be at the loyalty level at which this service is included.

Agree about the laundry on board the ship. Holland America line charged $35 for a full bag, it was a nice size, we fit a lot in, all rolled up tight. We were on a 2 week cruise this past May. I had a nice pair of black slacks which I wore on the dressier nights with a nice blouse. No one noticed I wore the same combo more than one night. I don’t worry about other people.

On the ship I never carried a purse around. I have a phone lanyard with an attached card case into which I put my ship key card. Never needed a purse, only when we left the ship at a port.

PS - I also have the Athleta Brooklyn pants, very versatile.

You say you have done this a lot. So you have intimate and personal information already about what to pack and how much. Seems you want to downsize your luggage. But what was it before? Are you going from XXL type stuff to Small, or are you only going from Medium to Small?
A carry on and a small backpack is the usual minimum hat many travelers use. For you to mention that seems to indicate that you are now packing way too much stuff and larger or more bags. Yet, it has worked for you on your frequent trips.
So, why now?

Nobody has mentioned a swimsuit and pool coverup. Add reef shoes to protect my husband’s diabetic toes. I suppose that I could wear the swimsuit in lieu of underwear on the plane but I bet that’s uncomfortable! Does no one go to the spa and/or solarium pool, especially on a TA when you’ve got tons of time?