I currently have four 5*+10 heroes already (Gwendolyn, Fae, A!Tiki, Nino) as a F2P, so I don't particularly feel the urge to merge anyone right now. Maybe in the future I'll merge Sheena (need more copies) and Draug (holding until Red Infantry Duel comes out, and if it does, Olivia is going straight to +10 in a heartbeat).
Currently I'm T20.5, although with Gwendolyn recently as a bonus unit I stayed in T21 twice. That might not happen again for a while unless this week's cutoff for staying T21 is 3790 or less. I'm consistently top 5000 in AA with scores usually from 5200-5214.
My favorite mode is Tempest Trials, where I usually grind 45000-50000 feathers just from HM alone during Lunatic 7 manually. (Yes, I really must be a lunatic, pun 100% intended.) It's no fun watching the game practically play itself for me. I realize that many folks like to use RD for feather farming, but in the long run I'll get more feathers by consistently ranking top 1000 in TT. Each TT event nets me about 66000-71000 feathers from HM, overall rank (10,000 feathers) and reaching 50000 points (11,000 feathers).
Outside of TT, I usually spend feathers for 1-2 new 5* heroes a month (+Spd Sothe is likely next in line) to give me more options in AA and PvE and to inherit new weapons 1-2 times a month (Fjorm with Slaying Spear+, then probably either WE!Lissa for a Slaying Hammer+ or L'Arachel for Blarblade+) and pocket the rest. Right now I have just over 260,000 feathers saved up; maybe I'll get to 999,999 by the end of June (or sooner).