We can help with this if you wish, it should be quite straightforward.
Though mind flashing Magisk and relocking is impossible, it’ll make your phone bootloop AFAIK. You have to chose between both.
There’s a detailed community installation guide:
More settings yes, more AOSP upgrades yes, but security is a vast subject. It depends how you define it. More privacy I believe yes, you won’t have the Google Apps always active.
Also, you’ll have a much lighter OS in the sense that it is quite sober and you can then install what you need.
Edit: Typo