How hard is it to get your frame exactly rightfor you. I'm thinking about the difference beteen my old raliegh - carlton and my new salsa caseroll (which if find inclined to wander). Could you give your frame just the right character? Has anyone designed an adjustable frame bike to use as template?
I guess it depends how picky you are. I used to make guitars. Classical guitarist where pretty picky often, and knew what they wanted, and why your guitar wasn't it. Steel string player would get something and comment on how it was different, and how that would work out great in this new piece they were doing, bring it a special sound. I mostly did classical guitars, which may be why I no longer do...
I built my first big crafty thing, a canoe, over thirty years ago. I know boats. I designed the canoe, and while I know some things that are wrong about it, I also by luck as much as anything, nailed it out of the gate, and really don't need to do that boat again. around twenty years after I designed the first I got the idea of trying that exercise again, really as much about just having another boat. But sure it could be improved, just not worth it.
When I was making golf clubs, the first shaft, the redoubtable Dynamic regular medium, was recommended to me, and was perfect, so that was a leg up. I could try everything else, but at least I had a basis for those further experiments.
I find occasionally you get stuff pretty nice the first time. Or you get it right the second time sorta on the rebound. I find a lot of radical ideas can slow a project down. Sometimes they are needed, but they can also just hold up the progress to the first real attempt.
While there are endless things you can do, at my weight there are fewer tubing choices than I would have thought. And I purposely try to keep it simple. Work on one aspect of what I want, rather than change everything at once.
I don't know about adjustable bikes, but there are adjustable frame fitting stationary deals, it would be hard to get the feel into an adjustable bike. I don't know that I need that. I just go off the position I established on existing bikes, and freeze that into the main tubes of my bikes, and play with non-fit variable. Some people are way wrong fit wise, but if you ride a lot, and listen to your body, then hopefully you have a reasonable position. From there I logic it up on a custom so it's in the frame and not the bolt on stuff.
Depends why it doesn't steer well. It could be general misalignment, which can be corrected to some extent, Park has tools that some shops may know how to use. better if it isn't built in in the first place, because with triangles there isn't a great way to get it out.
If it just has a more twitchy front end than you like, you could dabble in playing with different forks, though it can be difficult to find any stock ones that diverge from fairly standard numbers.
There are also devices that calm the steering action, and they are cheap.
I just looked at the geometry, and I would think that bike would be fairly stable. There would be no problem in building a frame that matched the carlton, but modifying an existing frame would be more difficult. There are adjustable frames for fitting, but it's rather difficult to make one to ride. One of the projects on my list is to make a fork with multiple dropout positions, but that doesn't allow head tube angle changes.