This garage door can probably be opened with a flipperzero brute forcing the code. It’s probably worth the bullet just to remove that vulnerability or some random RF noise opening it.

Cable modem? Make sure the coaxial connection is tight and any others you have.

It depends how many different streams will have running. 40 mbps is probably needed per screen or even more but 40 is absolute min for 4k. If you want to have enough room for people to stream and room for anything else they want to do you need 300 mbps minimum.