I think this can be easily solved. You don't need to raise XP for quests, it would be enough to slightly increase the final heroic xp - because heroic quests are discussed - and, thus, increase xp/rank. For some unknown reason, from 15lvl it grows the fastest and is the least needed for the next rank - only 2k xp/rank. According to what you yourself noticed at levels 9-10 and even earlier because from 7 is needed as much as 6k xp/rank. Only between 1 and 2lvl is less needed than from 15lvl upwards. Which is a bit absurd because for these levels quests are really quite a lot. I recalculated quite a bit in Excel, made two simple diagrams each, aligned the data so that it was more linear. The result is that the final xp is higher by only 200k xp, although some players will probably eat me for this idea, I raised the xp for rank from 15lv upwards. Well, and I leveled up a bit on the low levels from 4 to 7, because there are a lot of quests in that range as well. Of course, I refer all the time to the xp table for TR 2+ and this is just a confident suggestion
An additional plus would be the lack of penalties for entering a 20, 21lvl character on heroic 18-19 elites. In the past when from 20lvl you got full ED maybe it made sense. Now when from 20lvl we have barely a few points in the ED tree which won't give us much - core = hp/sp, power + some skill or feat/spell DC it's actually not much different from heroic enhancements. So saying that a 20lvl character is OP for heroic elites is an absurdity of supernova power. Especially since these heroic elites from 18+Q can be much harder than epic-hard quests from a base level of 20.