Some of those requirements are mutually exclusive. If you really want to burn multilayer discs, I think a Pioneer drive is the only game in town. If you only want to burn single layer discs, then other drives will also work. But do you really, really want to burn any optical discs?
Generally speaking, Pioneer drives can't be flashed.
Pioneer firmware update February 2023 A recent Pioneer firmware update made their drives incompatible with
LibreDrive which is necessary to rip 4K UHDs. Drives with older firmware still work well, probably the best, for ripping (& burning), but once the drive has the new firmware, you can't go backwards. A
seller in your region can still sell you a Pioneer that will work, however.
Needing it to be slim cuts down your options quite a bit. Lots of folks end up with full sized 'internal' drives in an external enclosure connected via USB.
If you want a slim drive that can be flashed, something like a LG BU40N could work. Pioneer has some slim drives as well.
Billycar11 recently made a post about drive quality rankings:
https://forum.makemkv.com/forum/viewtopic.php?p=154264#p154264
And as always, there's the
Ultimate UHD Drives Flashing Guide.