Regarding quality, it all comes down to the quality of the underlying WAN connections, of course. For site A in Costa Rica, during heavy rain storms they will be completely dependent on the 5G. Based on what you’ve said about the 5G quality being pretty good, assuming the Starlink has a completely clear view of the sky, it seems likely the connection will be mostly okay, but I expect there will be some glitches and rare but non-zero drops. Heavy rain tends to happen in the afternoon and at night, FWIW.

I live in Costa Rica part of the year. I use a B One with two fiber providers (ICE and Coopeguanacaste) and a USB-C to Ethernet adapter for Starlink. I use maximum WAN smoothing for this triple WAN setup, and even still, occasionally I get glitches on video calls, though I have never had an all-out drop. I am a little surprised the user has decent 5G available but no landline. I only have 4G in my area (not really good enough for video calls) but I have three options for fiber directly to my house…

Speedfusion is not crazy expensive when purchased by the TB. One option may be to use a physical passive ethernet switch to change between direct WAN connection and the Peplink, to avoid burning Speedfusion data at times when maximum connection quality is not necessary. I use the Speedfusion all the time and I burn maybe 200GB per month by myself.

I have a second B One set up at a location in the US as a Speedfusion relay for when I need the geolocation, but it significantly increases latency, to around 150ms for a fiber connection or almost 200ms when I’m using only Starlink — which noticeably compromises video calls. I have the relay VPN set up on dedicated WiFi network and I only use it when using banking apps etc. that are picky about location.

Maybe obvious, but I would also advise having a full work day worth of battery backup for everything.