What a complex problem... like most of you, I get extremely turned on at the anticipation of anal play, and a hard-on is not a problem. Then, as I slide down onto a plug of substantial size, there is a temporary increase in the quality and size of my erection, and then without more stimulation, it tends to subside... not disappear. Small plugs do nothing anymore, and extreme anal stretching certainly renders me limp, but full. When we are into extreme play, stretching and fisting or riding a giant challenger toy, it's all about the stretch and orgasms cum regardless of what state my cock is in, although usually it is swollen but not fuckable. I have found that ED meds help to overcome the issues, and lets us both enjoy a good romp in that in-between place...
With my standard butt plug, I get sufficient anal stimulation to greatly enhance any other erotic play we are enjoying, and with just a little encouragement, and a cock ring or two, stimulation quickly results in a rock hard cock with a butt plug firmly and deeply planted inside! It took a while and some great times experimenting until we found the right combination of depth and circumference for this plug. But it is worth the effort. I will say though that when we fuck and I cum with the plug inside my ass... it causes me to have a retrograde ejaculation, which means most of my semen shoots back up into my bladder instead of pumping out of my cock. That's because the toy is wide enough to really apply some pressure on my prostate. Next pee afterwards and out pours my cum! Not harmful, just not as explosive and impressive!
My advice, take your time, get comfortable with a flaccid cock orgasm because it's all part of the game, and find the right size plug that gets your motor running but allows for your cock to go along for a nice hard ride! And get cock rings... the great equalizers!