I'm using Esperanto in daily life where I can - so, translating my shopping list.

Mi estas vegetarano, but I do buy soy-based meat replacement products. How would I refer to these?

falsa kokaĵo?
pseuda kokaĵo?
imitaĵon de kokaĵo?
imitita kokaĵo?

In English I write, unseriously, "fake chicken"...

I tend to say "malvera kokajxo" but frankly your suggestions are all better than that; mine is a direct translation of "fake chicken", which is also my usual English expression. If you know what it's made of, you could also say, "soj-kokajxo", "glutena bovajxo", for example, like we also say "soj-lakto". "Vegetara/vegana kokajxo" is an alternative.

I say this is a great time to use your creativity (the creators of such products certainly do - "chik'n", "chickenless strips", etc.)

pseuxdokokajxo is a nice one. Perhaps I will start to use it myself!

"sxajnkokajxo" is something else I have heard. I vaguely remember reading it somewhere, perhaps on this forum, so I can't claim credit.

Ŝajnkokaĵo: "seems kind of like chicken".... I like it!