I also need to know on the 1st building you have a switch with an SFP(+) port or not, otherwise you’ll need a switch. Don’t use media converters. 300ft+ is only going to be good with fiber, copper will cause issues since you’re very close to going over spec.
Discussion
You're right. It's copper, currently, on a daisy chain (we have a third, small building between). The first 150ft is working great at full gigabit speeds, but the second got downgraded by the hardware to 100Mbps 😡 Same cable, and it ran at gigabit on install. The run is out-of-doors, so I got direct-bury grade copper lines.
I was 🤏 this close to getting a couple media converters just for that 300-ft line about a month ago (copper → MC → 300-ft fiber → MC → copper). The modem is copper-only, and I don't know if the ISP would like it if I swapped it for my own (including that I'm not sure I could get it set up because of vendor locking by the ISP).
Media converters work until they break and you have no diagnostics