I have done the Eurostar (Thalys) to London and/or Brussels several times to catch a flight home. I do go the day before and I don't wait until the FINAL train of the day.
In several years of doing this I have had ONE problem. During the "Brexit" mess it was taking 4 -5 hours to get through immigration at Paris to go to London. Now I had time to wait, but I was solo and standing there for 4 hours without any way to take a bathroom break etc...was not appealing so I developed Plan B. Thalys to Brussels and Eurostar from there to London... Didn't turn out to be as costly as I expected since when I sent Eurostar my refund request they "accidentally" refunded the entire ticket price :) Now on that same trip there was a woman in line who was sobbing hysterically and begging the exhausted and frustrated staff for help. By the time we cleared immigration and got to a train her flight from LHR would have left.. They would not move her up in the line and I understand why...it would have stared a riot!
You are are of course going to get the "expert" advice on why you entire travel arrangements are 'wrong' (from people who have no idea if they are or aren't. I know mine are not LOL!)