I've also had people question the agreement which raises a red flag for me (similar to having common area video cameras installed - what are they trying to hide ?). 

I have set us up with remote locks from Yale so the guests get codes, have a handyman that we trust on out team, upgraded our outside cameras, and are interviewing our cleaning team and doing all the things to prep us for doing this remote. 

Besides having a reliable cleaner and handyman, put in a smart lock, smart thermostat, and a Ring doorbell or other outdoor camera.

Major changes included, 6 apt rehabs (1 complete gut), foundation repair, serviced all 5 HVAC systems, plumbing, exterior paint job, new lighting throughout, wifi cameras and thermostats!

Can anyone recommend certain cameras, or a device to put on the door in case someone tries to force entry? 

If it is an exterior camera have you given the guest view access?   

They broke into locked closets and took expensive camera and dive equipment (note: don't put anything in your STR of high value that could easily be taken) trashed the place as well.  

Granted you could use documents and cameras to protect you.

Online rentals, security cameras, etc.

I have a wifi camera on the entry/driveway of each home (used to be ARLO, now WYZE) A wifi connected front door lock, Honeywell wifi thermostats, PHYN plus smart water device and leak sensors near the boiler.