For a flat bar conversion I don't know of a nicer shimano shifter for 11sp but I wouldn't have problems using this
https://www.microshift.com/en/product/sl-r761-2/ if shimano equipped. Although I'm not big on 1x systems for a tri I can't imagine that a 1x system won't be more then adequate and probably easily available. The courses tend to be flatter and a 48 or 50 with an 11-30 should have plenty of speed and the ability to climb most things on a tri course. The shifter I listed is meant for shimano road, I'd buy a small riser bar, toss on the shifter and get a set of mtb levers to match the brakes.
Having the seat too low will be the biggest struggle to overcome, if its a comfort getting on and off issue it might be worthwhile trying a size smaller to make it easier to get on and off. More then anything proper saddle height over longer distances is a real comfort issue with the knees and too low can really slow things up.
No matter what, this sounds like an ambitious goal and Chris sounds like quite the accomplished rider. I wish him continued success and blessings on his endeavors.