This right here. I liked how TNG did it. Series premier bring an oldster in to launch, maybe have a special episode or two with another.

If we really wanted Colm back, have it in the premier of Starfleet Academy where the new cadets are going through a hall of distinguished professors and have an elderly O’Brien do a cameo with a sample of one of his lectures. Nice to connect the show to lore and nostalgia but short enough to let the new cast stand on their own.

That said, I agree with Colm. Let O’Brien stay as he is. He had a perfect send-off.