very few stations are in two distinct areas. Gare de Lyon is one of those, and there is a bit of a walk between the parts. But nothing goes up on the screens until about 20 minutes prior to departure. So that's when I'd aim to be there. Wait for the platform to be posted and make your way there, following the easy to follow signs.
You already have a seat reservation so you will know the car number and seat number. Walk down the train to the right car (they are in order, if the train is a double set (two trains coupled together) the numbering sequence will jump. Just follow it along. Get on the right car and find the numbered seats.
An hour is 40 minutes' overkill.
The Thalys is even easier. At Gare du Nord all the tracks and platforms are in a row, sequentially numbered. Again, find the platform and walk down to the right car. Thalys used to have a staff member at every door showing you in after they checked your ticket. Post covid I don't know how they are doing it, but I would never get there too far ahead.
If you are new to trains give yourself a few extra minutes for de-bogglement. Plenty of snacks and drinks about for your nibbling pleasure.