Train from York to London then Eurostar to Paris

I have done this. I would say arrive at Kings Cross 60-90 minutes before the Eurostar departure time. So for a 13:30 departure, arriving at or before 12:00 is plenty OK.
You appear to be happy that Kings Cross to St Pancras is literally "across a road".

Rail Europe UK ( https://raileurope.co.uk/ ) will sell you a ticket for York to Paris as one purchase. Their times include all the necessary time at St Pancras, and you are then covered if the first trains is late.
For example, for a random Wednesday in March, it gave me a cheapest price of £81:

10:32 depart York, arrive 12:40 London Kings Cross, £23.50
Transfer & check in 1 hour, 42 minutes
14:22 depart London St. Pancras, arrive 17:51 Paris Nord, £57.50

Edit: The Eurostar site will also sell you tickets York to Paris. That site can be temperamental. The first time I tried it denied York existed.

What you are proposing isn’t a journey that I would be comfortable doing! The worst delay that I have experienced on the London - York train line was over 2 hours when there were signalling problems.

If all goes well, you need to allow 60+ minutes at the Eurostar terminal plus allow 10 minutes to get from the York train platform to St Pancras.

I would look at flights to Paris from Leeds-Bradford airport with Jet2 or Newcastle to Paris on Air France rather than travel via London.

You can book one through ticket from York to Paris on the UK Eurostar site. That automatically works out connection times. Leave York at 10.32, arrive at Kings Cross at 12.40 to catch the 13.31 Eurostar.

But of course, you have to wait until the LNER Advance tickets go on sale, and therefore miss out on the very lowest Eurostar fares.

The Man in Seat 61 outlines solutions to get connecting tickets if the York to London train is late.

https://www.seat61.com/UKconnections.htm

Essentially, lock in the cheap Eurostar fares now, then use raileasy.co.uk to buy an Advance ticket (when they go on sale) from York to London International CIV. That means if the York train is late and you miss the booked Eurostar, you are put on the next available Eurostar at no cost.

Thank you already for the great responses to my question. I did read the information on The Man in Seat 61's website. He has great info, but a lot of details to sift through! Thanks 'ramblin' on' for bringing up the London International CIV ticket. That seems to be a good option for our situation to help us lock in the low Eurostar prices now, but then add some safety for a late train from York.

I know we could do a short flight from northern England to Paris, but I try to avoid airports/planes if possible. I prefer riding the train and I arrive much more relaxed at my destination - and being able to bypass the journey from the airport into central Paris is another plus for me.

Having experienced delays on British trains, I would plan arriving at least two hours ahead. If your train is late, and you miss your Eurostar, the cheapest tickets won't allow changes or refunds. That can get expensive.

If your train is on time, there are plenty of places in either station to grab lunch and relax.

The London International CIV is the way to go.

Yes, sometimes delays happen, but actually not usually for long and not always. The CIV ticket will protect you.

You might want to consider how early you and your little crowd get up in the morning, and if you would like a bit of a break between trains to maybe walk about a bit and perhaps consider a lunch in or near the two stations in London.

Or will you shop at the Waitrose or M&S for a Eurostar picnic to have on the way through the Kent countryside?

Or are you going for Standard Premier or higher where on-train minions bring you refreshments and nibbles?

Thank you for this helpful information. Will be invaluable for upcoming trip.