Holy c*ap, 20 to 50 devices ??? Let's see now, everyone has a smartphone but the majority of people who rent on AirBnB do not have computers or tablets. So 20 to 50 devices would mean on average 20 to 50 guests ... which then would mean that you should be rolling in dough if you have that many guests ... and that in turn means that from what I can tell, you're juszt being very cheap with your unwillingness to add mesh extenders. Sorry, I know that's a bit rough, but c'mon now, really? 20 to 50 devices and you don't think that you should invest a little bit more for better access?
Might it be cheaper to just get another ISP account with a second or even a third router, followed by creating 10 - 15 guest accounts per router, which can then be spilt up into groups of user accounts for varying numbers of guests? Sure, that's a bit of a PITA, but it would only be a one time thing. Once everything is set up correctly, that's it and you're done. Seriously though, with that many guests money for a little re-investing shouldn't be an issue (methinks).
PS: The problem with extenders is that each extender isn't a great solution (we've experienced numerous problems with them) and that they need to be located close to the source that's using the extender, without being too far away from the router. Mesh would indeed be a better choice.
We use a Passthrough Powerline AC Extender which provides power WiFi power literally through the electric wall outlet, (Extender must be on the same circuits as the electric fuse panel of the residence with the router). These are totally reliable. It has LAN Ports as well as Wireless access. But because of the cost and the fact that the extender needs to be pretty much in the same location as the guest suite, we have that device locked into cabinet in order to make sure that it doesn't get stolen.
And no, why would you ask your guests to provide their own WiFi? I've never heard of any host doing that. It's a service which helps to bring you customers so it obviously benefits you to have that service! And again no, why would you roll the cost of hardware devices over to your guests? Hardware devices are generally a single time cost ... an investment ... which benefits you and helps you to make more money with more guests.
Once you get it all set up, you're finished.
Why should guests continue to pay for that?
Wouldn't it be possible that by raising the price you'll end up with fewer guests?