@chad would have to give you the best advice on the duathlon side. I’ve only ever raced one du, and it wasn’t draft legal. I imagine a lot of the same principles apply, but obviously in different ways. You’re just going to have to be a strong runner, period. That’s always a part of draft legal multisport since most races are going to come down to the run unless someone can just completely demolish the field on the bike.
There’s probably no way around your power generation other than to work on the TT bike. That said, most riders don’t see that big of a dip between road and TT bike… have you had a good TT fit where the person was focused on your ability to put out power? A lot of fitters (wrongly, IMO) prioritize your aero position, but you’re talking about a 20%+ power reduction. You’d be better off slightly less aero and able to put out closer to 290-300W on the TT bike. I think the TR guys have talked about a 10% power difference between road and TT as somewhat “normal”. Yours seems way more than that!
The last bit would be to know the field if you can. If you know there’s one uber-biker out there who is close to you in terms of run pace, sit on that guy’s shoulder on the first run and make sure you get with (or ahead of) him coming out of transition. I’d say if I were racing a draft-legal duathlon here, I know the one guy I would sit on during the first run, and then I’d work with him or sit in his draft on the bike (he’s a beast), and then beat him on the second run. We raced each other in the one duathlon I’ve done - it wasn’t draft legal, unfortunately… I put 15 seconds into him in a 1.5mi run, and he still beat me by enough on the bike that I couldn’t catch him on the subsequent 5K run. If it was a draft-legal event, it would’ve been a different story!