Partial underextrusion in walls
Localized underextrusion usually means you're oozing material somewhere it wasn't supposed to go. Since you have Marlin 2.0, linear advance might help solve that, and in general right retraction amount is important and turning off combing may be needed (combing over infill allows material to ooze).

Also there are reported bugs in Marlin 2.0 such as  https://github.com/MarlinFirmware/Marlin/issues/15473 that might be related, though I would think you'd not be using junction deviation since it's supposedly incompatible with deltas.