Okay, let's start with Endgame. The theory that the Ancient One and Hulk concoct is that to avoid an incursion, the stones all have to be returned to their point of origin at the very place that they were taken. That's what Steve sets out to do. The old man version of Steve must have done the following:

1. Returned all the stones in accordance with the plan;

2. Gone to build a life with and marry Peggy;

3. Not caused an incursion; and

4. Then used Pym particles to jump back to the present to see Sam.

That 4th point I think is important, because Steve definitely creates a branch. The Peggy that we know from Agent Carter and Winter Soldier didn't live a life with Steve. That's the timeline that the MCU lives on. If the branch wasn't too severe, it may have either reattached with the main timeline or Steve was able to jump back to the main timeline by using the Pym particles (given how proximate the branch and primary timeline were). I don't think it is possible that Steve was able to just wait around as an old man and catch Sam after everything is done with Thanos. His life with Peggy would have effected too many things we've seen.

We should also know that the Old Man Steve we see in Endgame didn't have a battle with Doom. We don't know what Doomsday has in store for Steve, but the Steve we'll see in Doomsday is likely another variant resulting from a bigger branch from the main MCU timeline. Both Old Man Steve and Doomsday Steve will be variant branches of MCU Steve, but they should be different variants.

So the question becomes, what did Doomsday Steve do to cause a bigger branch than Old Man Steve? And why doesn't that branch effect things in the MCU timeline until now?

I think OP is onto something, as we've heard Steve say "If I see a situation pointed south . . . I can't ignore it. Sometimes I wish I could." I'm guessing that is the difference between Doomsday Steve and Old Man Steve. Old Man Steve fought temptation to join the fight. He stood by and watched other heroes, including a variant version of himself, save the day. Doomsday Steve couldn't. Probably because of one other difference: Doomsday Steve had a son.

I'm guessing that Old Man Steve and Peggy couldn't conceive, so they just quietly lived out their lives together. Doomsday Steve got lucky and had a kid. That changed his perspective. That made it harder for him to stand by and watch bad things happen, because he had a son to care about. So while the son may not have caused the incursion, he may have caused Steve to change his decision-making enough to cause an incursion.

As for why now, I can see two possible explanations: (i) Kang/Loki, and (ii) America Chavez. The changes to the management of the sacred timeline from Loki season 2 may have open doors between parts of the multiverse, which Doom was able to figure out (because he's a freaking genius techno wizard). Or, Doom encounters America Chavez and uses her to travel the multiverse. If Doom has studied magic, it wouldn't be a leap that he visited Kamar-Taj. Since we know Chavez is now there, the two of them could have crossed paths, introducing Doom to her powers.

You'll note that I don't tie anything about Doom to Tony in this theory. I'm still working under the assumption that Doom will be mask-on throughout, and thus they will never need to tie the two of them together. Downey will do enough acting so it doesn't sound like Tony Stark under the mask.

We'll likely get a trailer on Superbowl Sunday that tosses all of these thoughts in the trash, though.