Before spending another dollar, Euro, two-pence, whatever on kit, it might well be worth working with a good fitter using a highly adjustable fitting bike tool. There are different tools available but something like the Retul MUVE or GURU Fit System for folks wanting to google search.
You could have the fitter put you in different positions, check your range of motion limiters and determine what position(s) will be rideable or not. It’s no good wanting to be aero and spending money achieving a position only to find out you can’t ride that position. Once you know the position numbers you can then find the right frame and contact point kit (Frame, bars, stem, seat, post, pedals, crank length) to get you into position.
Further, don’t go from one extreme to the other. Tall folks and older folks often have joint issues, ROM and flexibility issues. If you go from upright to slammed all in one move it is asking for physical injury.
If possible, work with someone who doesn’t have a stake in selling you a new bike and spell out your project in advanced as it might take multiple sessions.
Personal anecdote, there was a time when I was seriously chasing aero for TT’s. not because I’m good at TT’s, but was dedicated to doing the best I could. Went to A2 Wind Tunnel and worked with Mike G. Mike is a fantastic fitter and really knows Aero and fit. We sorted out a couple different positions. One was extremely fast, but exceeded my ROM. Another was more relaxed but still very aero. I ended up not being able to use the extreme position for full season use, but have it in my hip pocket should I ever want the most aero possible. I went a notch back in position, still very good, and can ride that all day long no problem.
My take is one can spend a lot of time, money and frustration stumbling through the wilderness getting sorted on fits and aero. Or, one can access the right tools and expertise and get to it more quickly and confidently. Caveat is that chasing this type of stuff can lead to some expense upfront because accessing the right tools and someone’s expertise isn’t free. That’s one of those personal choice things but I’ve found getting to the best choice faster can also be the most economical in the long run.
$0.02 worth!! Fun thread.