It’s a bit like having a compiled executable. Yes, you can technically modify an STL, but you’d be much better off if you can edit the source that the STL was generated from.
> During COVID lockdown I learnt 3D modelling via Blender and so STLs are my lifeblood for 3D printing!
Blender is a good skill to have, no doubt. 99.9% of all the 3d modeling / printing that I do is "functional" and you need an entirely different set of skills/techniques/tools to build a functional part compared to the models that blender spits out. At the end of the day, all techniques do generate a list of triangles but "going backwards" from that list of triangles can be a massive pain in the ass especially if you have a precise modification that you need to make.
If you want to share your models _and_ make life easier for anybody that's going to tweak them, STEP files are the best bet.
During COVID lockdown I learnt 3D modelling via Blender and so STLs are my lifeblood for 3D printing!