Good points!
Yeah the endgame definitely becomes stale and will get some improvements.I'm thinking of tuning the drop rates down for the highest rarity items to lengthen the gearing process a bit at least. And all the enemy stats were supposed to scale the same but it seems their damage is somehow bugged. and doesn't. And at least the boss (and NPC) bullet patterns always just do flat one damage meaning they (especially the traders) just become free loot/score piñatas that tickle you a bit.
A way to trash/transform some of your letters sounds neat and a way to go. Maybe even something as simple as you giving some about of letters and you'll get 50% back as random letters. And there definitely is too little vowels (A and E mostly). Even more noticeable for set bonuses as I don't think there are any vowel weapons except for Y. Could also add a "auto move" with right click or something that would move the letter from stash to the first unfilled spot. Same could work for equipping stuff too so you could just right click to replace the currently worn item.