My suggestion is for Reaper mode to add a new bonus which would give players additional encouragement to encounter and fight the reapers themselves:

A bonus of 10 reaper xp per skull for each reaper defeated. Double this bonus for "special" reapers (vengeance & despair) in R6+.
If it helps performance, add it immediately to all players upon reaper defeat. Otherwise, add it upon quest completion.

Examples:
- R2 with 5 reapers defeated = 100 bonus reaper xp (10 x 2 skulls x 5 reapers)
- R7 with 5 regular reapers defeated = 350 bonus reaper xp (10 x 7 skulls x 5 reapers)
- R7 with 3 regular reapers and 2 special reapers defeated = 490 bonus reaper xp (10 x 7 skulls x 3 reapers) + (20 x 7 skulls x 2 special reapers)


Thank you for your consideration.