

combine announce URLs into one torrent file
No.
or create a separate torrent file for each tracker?
Yes.
You can also check in the rules/FAQ of each private tracker but it’s universal that all private trackers require their torrents to exist and announce/share peers separately. That doesn’t mean the data has to be separate e.g. if it’s the same torrent data you can point multiple torrents to the same data.
Nope, I prefer being able to run my own network router, open/close my own ports, block ads on the network, hopefully get as much bandwidth as I can, etc. so it’s usually better for me to subscribe to my own internet.
… But since you bring it up, coincidentally I currently live on a street with shops/restaurants on the main floor under me. And all their wifi networks are visible from my apartment… so technically yeah, if I go through the trouble of collecting all their wifi passwords I could just hang out on their networks for free internet. Internet probably wouldn’t be great and not very private without a VPN but for free web browsing it should work.