Simple solution is often times the best.
instead of increasing this bonus or adding a stacking bonus or adding higher slayer pots or different XP pots, why not take the simple solution?
Lower the XP needed to get to 20 and forget all the new bonus.
TR1 needs extra... +25% over non-TR (or 2.4ish mil XP)
TR2 needs extra... +50% over non-TR (or 2.85ish mil XP)
sure the people that have spent lots of time getting completionist already might be angry, but it would make a lot more people happy.