Should start a new thread about changing round wounds to flat wounds...I would've thought it to be a straight forward switch! Been eye balling the new 5-string solid bodies, but they only come with round wounds and I know I don't like them. Figured it would be an easy switch to flats whenever...
Wow, out of my price range, but I'd love to have one...
https://kalabrand.com/collections/solid-hawaiian-koa-top-u-bass/products/ubass-koa-15th-fs?variant=41107820281926
Assuming the stock strings are Pahoehoes, yes they are different than anything else and need some stretching, but they shouldn't buzz. Try tweaking the truss rod. Flatwounds will probably require lowering of the action and setup work.
Nice! I would use Road Toad Pahoehoes, for a nice, easy, soft feel and that mellow upright sound. That's how the Ubass was designed. They are kind of a pain to install, but well worth it. There's a video on youtube that is very helpful.
The EM-FS isn't solid mahogany, it's laminate. They both use Pahoehoes, probably. For the same price, get the old solid mahogany! Older is better, IMHO.
Actually, I don't think the EM-FS is fretless...