[Question/Technique] Cooking down berries, and getting seeds out?

I'm doing a raspberry trifle this year for valentine's and last time I made it the main problem was the seeds in the sauce. While some are unavoidable I'd reckon, it was my first go a cooking down fruits into a sauce and I'd like a smoother reduction this time.

I will have time between sauce prep and assembly, so if it's just a straining that takes a while with hot/thick/sticky that is fine.

Thanks!

bon appetit