To jump or not… what are typical equity valuation multiples after 3-5-7 years?

One successful fintech startup may have 30x the valuation of another successful fintech startup. So looking at the median or average multiple won't help you at all.

That said, I recommend going through this list:

https://docs.google.com/spreadsheets/d/1akR9F_JfVEvOleXn_mUB9hKmmtm5-1V8iQmhAWTqTc8/edit

For each startup (or for a sample) look up what they raised as Series A and Series B and how much time they had between rounds. Try to make an assessment of the percentage of startups in that list that have a better business model and founding team than your startup.

You'll probably conclude that joining the startup doesn't make any sense financially (especially when you calculate your risk-adjusted return as opposed to the expected value) but startups are always an adventure so it's alright to throw caution to the wind.

The rule of thumb is a growth of 5% each year. That includes 1 company growing with factor 50 and many others collapsing. If the company was already quite successful in the past, that does not guarantee a steady growth.