Honestly it feels kind of random whether a pair needs amp/dac or can be plugged straight in, but like the other guy said, you need a low output setup or it'll get worse and ground noisey

Ear tips are very important and a pretty personal choice. I have huge ear canals too and biflanged tips make a difference in terms of grip and sound quality for me. You might want to try them

I think that just depends on the headphone. It really does feel random how much support a pair needs. I wore my FH7's outside for the first time last week as was disappointed severely (I have no mobile amps). They cost over 300 and sound garbage without support just to be blunt. So maybe look it up for any pair you want to get and see if reviewers agree on the support it needs