I spent a good couple of years and a lot of money when I started getting into this world buying and trying libraries from all the heavy hitters. I resisted CSS as I didn't initially love its sound. I did however eventually take the plunge. Now having been working professionally for a few years, the basis of my orchestral template is CSS,CSSS,CSW,CSB and True Strike. This is what I use for 80% of my orchestral writing.
I have bought Project Sam's new linage percussion, whilst I haven't spent very much time with it, it seems a pretty good all rounder.
Setting up a template is pretty easy with CSS etc...You can get lots of advice Here, or there are even some premade ones available to buy. It's not hard with a bit of help.
I don't have BBCSO, but I don't love the other spitfire libraries that I have. Mostly not very consistent in terms of dynamics/layers and the intonation on SCS is terrible.
CSS & Project Sams long term support for their libraries is unparalleled. Project Sam are still making free updates to libraries that came out over Ten Years ago. Same with CSS, they are real musicians who really care about their products.
Good Luck