i am using my EOS M (1st gen) camera far more than my 5D3. i find ergonomics better on the tiny EOS M, because it has a good touchscreen that is fairly well integrated into overall user interface. longest lens i am occassionally using is tamron 150-600, also preferably on EOS M because of UI advantage: i AF and make all settings on responsive touchscreen and then release shutter with small Canon RC-1 remote trigger (IR). with large lenses i would not need "any UI, physical controls or menu" on camera, i'd prefer bluetooth and a decent smartphone app to get things done. looking forward to trying out my daughters new M50 (wifi, bluetooth, nfc, flippy touchscreen) once i can get my hands on it.
i never handhold a 600mm lens, not even a relatively compact, lightweight one, although i have a stable hand and decent shooting technique. long tele lens always goes on tripod with me, perfectly balanced, remote triggered, and camera body can and shall be as small as possible also for that use case.
handheld, i will be using ultra-compact f/1.8 to f/2.8 primes or very compact f/4 zooms (no interest in f/5.6 or f/6.3 lenses). i will also buy very compact, non-L, good IQ, affordable EF-x 16-35/4 IS STM and EF-X 24-70/4 IS STM zooms for new short FFD "slim" mount.