I use tf_medigun_autoheal 1
I used it this entire season. You want to know how much of a difference it makes? Absolutely none. My pocket Mustardoverlord (the single most heal-demanding player in the league) for 20 straight matches never noticed a single time. There is no "delay" when you switch healing targets; there is no penalty while you build uber.
The only difference is that you don't get carpal tunnel after every match from holding mouse1 for 30 mins straight.
If somehow you still have your tinfoil hat on about autoheal 1, you can do what sigma told me on his stream one day:
Sigmasin: alias "weapon" "alias +heal +attack;alias -heal -attack;-heal"
» alias "medigun" "alias +heal -attack;alias -heal +attack;-heal"
» then have something like "bind 1 slot1;weapon" "bind 2 slot2;medigun" "bind 3 slot3;weapon"
» and for safety you want...
Maxfieldtf2: yes I see
Sigmasin: -attack -attack2 weapon
» to undo w/e state you might get into