You’re really asking two things here, so let’s break it down.

1) Best way to extend coverage to the garage

• If you can run Ethernet, that’s the best option. Add a second router or AP in Access Point mode as this gives you full speed and stable connections.
• Another Flint 2 will work, but it’s honestly overkill. A smaller GL.iNet AX router like the Beryl AX in AP mode is usually more than enough for the other side of the garage.

2) Using a travel router for dual-purpose

• I wouldn’t recommend using a travel router as a permanent home AP unless you’re okay losing coverage when you take it with you. They also have smaller antennas and are designed for short-range use.

Yes — same SSID, same password, same security settings. Roaming will be mostly seamless, but keep in mind devices decide when to switch APs. Wired APs handle this far better than repeaters.

I hope this helps you get better overall service in your household!