Hot water at master bedroom shower was always slow since we bought the house but is now just a trickle. Cold water and all other hot water faucets are fine. I have an older house with plumbing that is behind walls, so instead of cutting walls do you think I should cut the pipe in the basement and try to back flush it to see if it is just clogged with rust or something?

so instead of cutting walls do you think I should cut the pipe in the basement

No, I think you should turn off the water, remove the hot water stem from the faucet. Then turn on the water briefly (about 5 seconds) and flush out what ever is blocking the hot water side of the faucet. Then reinstall the hot water stem.