I have OnePlus 7 Pro that I successfully flashed with LineageOS 21 with MicroG. Do you have some interesting apps or ideas to take advantage of it? I thought of some Magisk modules. Maybe someone is more experience than me! This is the spare smartphone, the main one is GrapheneOS, so I don’t mind breaking stuff.
Insular (the degoogled open source version of Island) has a page comparing Island, Shelter and Insular
https://secure-system.gitlab.io/Insular/faq.html
I don’t know if its updated, but you can pick the ones that fits your needs.
I picked Island because I thought I may have issues with my notifications if I had chosen Insular. (Plus, there was this bug I was talking about and thought that it would fix it. Installing Island wasnt the solution, but I had already spent a lot of time/days setting up my phone multiple times and didnt want to try it again, with the possibility of problematic notifications.)