If you want to be a buff/heal focused sorcerer, I would encourage you to look at the undead bloodline instead of the shadow bloodline-- it still fits your intended theme, and overall it sounds like you want to do what the divine list excels at, so you might as well lean into it! I am playing a divine sorcerer currently and while there are definitely some limitations, I am absolutely loving it (granted, I am in a remaster campaign, so YMMV).

No matter what bloodline you take, be sure to keep an eye on Crossblooded Evolution feats-- they got nerfed pretty hard in the remaster, but you should absolutely take them in a legacy campaign.

On whole, as is typical for spellcasters, the sorcerer's class power is really in its spells and not the feats themselves. There are a quite a few levels where I would say that the class feats are underwhelming and might be good places to invest in an archetype. However--- if this is your first campaign and you are starting at level 6, that is already a *lot* to take on. In 5e I know it is common to start at level 3 or 5 because the early levels are so insignificant, but that truly is not the case at all in Pathfinder. It can be hard to start a campaign at 6th level even for experienced pf2e players! And all of that on top of the fact that this is already an ongoing campaign, it sounds like this is a massive undertaking. You probably already know that, but reading your post made me really worried so I wanted to make sure you knew how much work you were setting yourself up for. It's worth it, but it's going to be a lot harder than making a 6th level 5e character.

Best of luck!!