Custom roms for OnePlus 3/3t with android pie.
Custom ROMs are very much a personal preference without knowing much more about your usage and wants, no one can advise you. Why not head over to XDA developers and peruse the multitude of ROMs available?
Most popular is resurrection remix and lineage os. I haven't personally used either of two.
Introducing the New User Ladder System
by Crystal Z., 2019-07-15