I am thinking of extending part of my trip and need insight. If I have a Eurostar ticket booked for 8:00 a.m. out of St. Pancras, would it be too risky to leave from Canterbury West early--at 5 a.m.--the same morning as the Eurostar trip to Paris? I had originally planned to stay close to St. Pancras the night before departure but would like to extend my trip in Canterbury if it is do-able.
Too risky. UK rail has been having problems and if there is an issue with your train you are in trouble.
Stick with your original plan.
I usually fly out of Gatwick, which is an hour away door to door including parking my car off airport and taking the shuttle bus. I never stay at the airport the night before my flight. However, I am in control of the timing.
The difficulty you have is that you are restricted to the times of the trains. I have just looked at the timetable for February (I know this isn’t when you are travelling) but having taken Southeastern trains into London commuting for years, I can say that there’s rarely any dramatic change in the timetable.
The first train out of Canterbury arrives at St Pancras around 6.20am. You need at least an hour for border control, so you need to be there at 7am absolute latest. So, it appears that you should be ok. However, if the first train is cancelled, the next train an hour later arrives after 7am, so you would miss your Eurostar train.
There are numerous train strikes at present and no one knows how long they will last. If you want to travel on a strike day, you are unlikely to be able to get into London for an 8am departure.
I would look to either push back your Eurostar train by an hour and stay in Canterbury or stick with the 8am train and stay in London for your last night.
For the Canterbury to St Pancras journey buy a London International CIV ticket which means if the Canterbury train is delayed you're entitled to be rebooked on a later Eurostar - more info in the link
https://www.seat61.com/France.htm#other-uk-towns-and-cities-to-paris-by-eurostar
There have been many posts here--and comments elsewhere online--to the effect that you are now advised to be at St. Pancras two hours before your Eurostar is scheduled to depart. Just yesterdays I read a comment on a different forum about occasions when even two hours wasn't enough of a buffer.