Europe Itinerary 6 months perhaps a bit longer

Lucky you! Assuming that the UK(does not include Ireland) is non-Schengen, I would keep it in reserve for some of your non-Schengen time. May-August cooler parts of Europe August-go to Scotland so you can play golf, then go to England, Wales, Northern Ireland for total of 90 days. Then back to Schengen zone.

Sounds like a great trip.
We have never been on that lengthy a foreign trip, although we have lived overseas for several years.

We did a great drive tour of Wales and England last October. Loved the countryside. Here is my review with details of our trip, perhaps it might assist you:
28 days in Britain and Celebrity Eclipse home
https://www.cruisecritic.com/memberreviews/memberreview.cfm?EntryID=599139

Going to colder places seems smart early in your trip, since you start in June. Renting a car in Ireland you are required to buy the insurance from the car rental company, so I am told. You might wish to rent separately for Ireland and the UK.

If you are not used to driving on the left, suggest renting cars with automatic transmission and a navigation system, unless you will have a GPS with the European maps. Try to rent small hotels or B&Bs near the city centers with parking included or your cost of parking a vehicle will be too much.

If you go to Scandinavia, its is great, consider taking a cruise of Norway up to the Arctic Circle for 10-12 days.
However, if you spend a lot of time the Ireland and the UK, you would have to go when it starts getting cold. I wouldn't go to Scandinavia from October on.
Scandinavia is expensive, but the people are great. Don't miss Denmark.

If you go to France, don't miss Normandy and the D-day beaches, American cemetery as well as the amazing Bayeux Tapestry.

Paris is wonderful, but get rid of your car there. There is a great high speed train from Paris down to Barcelona, unless you take the one to Lyon, France or farther down to the Med.

If you visit Spain, don't miss Barcelona, Madrid, Toledo, Segovia, Seville and Granada. Consider going to Portugal and see Lisbon, Sintra, Obidos and up to Porto. From there you could go to Santiago de Compostela, then the Basque region of Spain and France, then visit the Bordeaux region of France. You haven't seen Italy. If you have time, it is my favorite European country.

Count those days very carefully. Remember that both your arrival day and your departure day in the Schengen Zone count, so you must exit no later than Day 90 and return no earlier than Day 181 (if you exit precisely on Day 90). So you must be outside the Schengen Zone for part or all of 92 days if you use the full 90-day allotment at the time of your first entry.

I agree that, since the Schengen Zone is so large, it may be strategically better to begin your trip in that area.

And remember it doesn't have to be 90 in and 90 out. You can do it 30 in and 30 out almost forever. The key is to look back and make absolutely certain that in the past 180 days from today, you have not been in the Schengen zone for more than 90 days. Tomorrow you do the same. And next week. If you do that make absolute certain you have clear entry and exit stamps in your passport. Before going you should probably obtain a passport with extra pages just to be safe.

I have one of those extra-page passports. It is pretty doggone bulky in a money belt.

I'm one of those that prefers to drive--so I can travel more efficiently and see more places on my schedule. The best way for extended trips is to start and end your trip in France and lease a vehicle. You will save money, and get better insurance and terms for the time you're on the continent. You can also take it places where rental companies prefer you not take their cars--like to the east.
The Scandinavian countries are best seen June to the end of August. In September, the temperatures drop but the big problem is rain and the weather. We were there the first week of Sept. and had great weather--out of the ordinary everyone told us.
Our favorite places: Scandinavia for short, quick visits, as everything is deathly expensive. Copenhagen is great. We love all of Austria and Bavaria/Munich--easy to travel through and cheaper than Switzerland. Italy is a favorite destination for everyone. Our new favorite place is Budapest as it's very inexpensive and the young people are very hip and lively. It's much less expensive than Prague too. We loved our cruise to Malta, the Greek Isles and Turkey. We like Paris for it's beauty--usually short 4 day visits. We also like London, but are not so up on Ireland because it's pretty quiet. I'll stop there.

My wife and I are retired and take an extended trip to Europe each summer. With inexpensive European flights you can go anywhere not just nearby countries.

We usually fly nonstop Orlando to Gatwick on Norwegian to start. Last year we left in early May, spent a month in Ireland then a month in Scotland, both non Schengen areas. We rented a car in each country and generally followed RS guidebooks but added extra days, 2-4 nights in each stay.

Then we flew from Edinburgh to Vienna. 2+ weeks each in Austria, Slovenia and Hungary by train and car. Flew to Corfu, Greece and spent almost 5 weeks Island hopping by ferry until we flew back to Gatwick from Kefalonia. Great trip, five months altogether. I prefer to pre plan it all. Every nights lodging, all flights and car rentals are reserved before we depart.

So now I’m working on next summer, probably four months using a month in Cyprus as our non Schengen extra. We will fly to Gatwick and hop over to Amsterdam. Looking at Netherlands, Sweden, Finland, Estonia, Latvia, then down to Rhodes and more Greek Islands hopping with a finish in Cyprus. Really you can easily go almost anywhere.

Don't forget eastern Europe. Poland, Czech Republic, Croatia is absolutely beautiful, with great seafood. I've not been yet, but would love to spend some time in Slovenia.

Those are all very interesting countries, but only Croatia is outside the Schengen Zone.

From google: The European countries that are not part of the Schengen zone are: Albania, Andorra, Belarus, Bosnia & Herzegovina, Croatia, Cyprus (technically not Europe), Ireland, Kosovo, Macedonia, Moldova, Montenegro, Romania, Russia, Serbia, Turkey, Ukraine, and the United Kingdom. (Also but not practical: Monaco, San Marino and Vatican City). Remember that calendar days are counted, so entering on July 1 and leaving on July 2 counts as 2 days in the zone. So, for instance, going to Andorra from Barcelona for 3 nights only gives you 2 non-Schengen days, but uses most of 4 days.

May/June in Ireland/UK, then July/August in Scandinavia and Russia. Most of Italy and much of Spain have very mild winters, leave them for November. I'd want to spend at least part of December, though cold and snowy, visiting Christmas markets. Then the problem is where to spend a month or more outside the S zone other than Croatia and Turkey. Early October might be a good time for Croatia, Turkey could be fine even in early November.

I wouldn't count on using Andorra as an escape hatch without careful research. I've just done some quick Googling, and it has left me with the impression that there are usually no immigration controls at the border, so you're unlikely to have any record in your passport of time spent in that country, even though it is theoretically outside the Schengen Zone.