I've read that Lineage comes with the required firmware. That would be best, but I don't know if that's technically a problem.I think Oneplus 8T PE maintainer changed from Rohan Hasabe to Eliermi Cunha so maybe the PE 13 will get a separate XDA forum. But, indeed, I also wonder if OOS12 or OOS13 (or both) firmware can be used to install PE 13 on top of. I hope it no longer requires (as PE 12 did) OOS11.
boot dtbo odm product recovery system_ext system vbmeta vbmeta_system vendor
Official OOS 13 => OOS 12 C.36 (with MSM)