Each fan has a limit on the length of the duct run. There is no generic answer. Each elbow effectively decreases the total length as it slows the air flow down. There are standard values of how much an elbow adds to the resistance, but without knowing how much the fan can push, it doesn't mean much.
At least the fan from a full bath is normally run when trying to exhaust the excess moisture resulting from a bath or shower. This is NOT good to exhaust into your insulation. Running it out a soffit vent is akin to dumping it into the attic since the air is likely hotter than that outside, and will come back in an adjacent vent, cooling off along the way and adding moisture you don't want that could condense. You are right that trying to install ductwork in a joist bay that already has insulation in it and you have little access will be nearly impossible without either tearing up some ceiling or floor. The reason you have trouble finding soffit vents for a fan exhaust is that they typically aren't recommended. If you don't have an end wall where you could exhaust them, a good roofer could make a watertight penetration through the roof. Combining the outputs is problematic unless they are designed specifically for it; most fans are not. Fan-tec makes some that use one fan motor, remotely mounted, that can pull air from multiple locations. While maybe tough, venting through a stone/concrete sidewall could be done, but expensive.
It's a tough situation. Code requires a bathroom to either have an operable window or a working exhaust fan (to the outside!), so you technically may not be up to code and that could bite you when it comes time to sell. If you try to dump a lot of moisture into closed spaces, you may end up with mold issues.
Not sure what to do since I can't see what you have to evaluate alternatives. Keep searching, something may click eventually.