Don’t ever check a bag unless you absolutely have to. Shouldn’t be an issue for you if you’re just doing short hops in Europe. You’ll soon become an expert at packing what you’ll actually need on the trip rather than what you think you’ll need.
Aim to reach silver status with BA which will give you give you lounge access and generally a marginally better level of customer service. This is their sweet spot really, and gold and GGL status require lots of long haul flying.
I’ve generally had pretty good experiences with easyJet. Avoid Ryanair like the plague if you can.
Try to avoid excess booze and the crappy food folks tend to eat on the go if you can. Your waistline will thank you.
How many flights will you be taking per week? The sanity question is really determined by this number!
Good luck. I did lots and lots of flying for many years. Enjoyed some parts of it, hated others. Like anything you’ll carve out something that works for you