There are labs you can just order stuff from. I.e. I gave myself full bloodwork done at Synlab as a present for my 35 birthday, and they had covid antibody-levels test available as well.
I was lucky that I could use some corporate benefit for that, but even out of pocket it would be 30-50 Eur? And most things were within baseline, and things that weren't were close enough (I.e. I don't feel like stressing about cholesterol just yet), but I like I have a baseline to refer to.