I think your use case falls outside of what Meshtastic is capable of supporting. You say exchange text messages to coordinate professional development, but very quickly this can lead to someone trying to perhaps share some document? I don’t mean to discourage you from meshtastic, but I believe you will spend the money and realize it does not suit your needs. It certainly would be good to use it as some type of emergency communication link where no other communication means exist.
I would suggest you check out some of the Ubiquiti products for wireless networking. We have used Ubiquiti NanoStation M5 antennas to link remote oilfield wells that are anywhere from 2 to 12km apart. This is a high speed wireless Ethernet link. You can have a WiFi router plugged into it and many people can connect to this network. There are other models that provide even more range where required.
Check out this guide on Ubiquiti website:
https://operator.ui.com/
There are also many videos on YouTube on this topic.
I realize you are looking to do something on the cheap but I don’t see that investment in meshtastic for this use case will be of any particular benefit.
There are other brands other than Ubiquiti that offer similar product, perhaps cheaper. Reason I recommend Ubiquiti specifically is that units we installed 5-6 years ago are still in service, I don’t ever recall having to reset one.