rhm

I'm pretty sure all the things discussed above can be adjusted to where you want them.

Gearing shouldn't be a problem. chain rings up to 53t are everywhere, up to 62t only a little more challenging. if you were happy on an MTB with a 32t, I don't anticipate any issue you can't fix.

Small wheels rotate faster. We all get that. But does effiency suffer from this? I really doubt it.

So to your question, I'm sure you will find a way to get the saddle farther forward. Some saddles have longer rails than others-- you should look for one that can be cantilevered forward more. Doing thus you may risk breaking the rails-- but it's still worth a try. Some seat posts have more setback than others-- you should find one with zero setback. I realize you need a very long one, which will narrow your choices, but BMX riders use some pretty long posts, so you may find something.

Note also, shorter crank arms will help bring the pedals under your knees a little bit too. Putting a fat tire in the rear and a skinny one on front will make a difference as well-- but that's the last thing I'd try.

One way or another I'm confident you can get comfortable. One of my bikes is from the WW2 era and it has the 68⁰ seat tube you mentioned. i have the traditional saddle clamp, separate from the straight post, with the clamp in front of the post (whereas behind the post is more usual). this compensates for the frame geometry nicely. I don't know whether that's an option for you, but again, worth a try. You may even be able to use your existing seat post backwards.