I have gotten to the point in my cycling life that I want to do a semi-custom bike or at least swap out some components on my existing bike.

The choices are overwhelming and I am not certain why one would pick what.

As simple background, I mostly like riding for practical reasons (i.e. errands, visiting, going to the park, etc.). In the past, I have ridden for pleasure/exercise (fast loops and long rides) which I will likely not do again, and commuting, which I may do again. I don't want drops anymore and my current bike has whatever a flat bar with slight upswoop is called (look like wings to me). I am dyslexic so gears have always been confusing for me as my brain processes things like that in reverse but my mind sees things forwards (hard to explain if you haven't lived it). I am tall. I currently have a Terry Valkyrie Commute but I have been pining for a step-through / mixte frame because as tall as I am (5'10" 35" inseam), the seat is pretty high up and I would like the option of not swinging my leg over it all the time. I may opt to just change some components on my Terry rather than trying to change to a whole new bike and live with the cross bar height for now. I want the ability to rack the bike, fenders, etc. My current bike has that.

My wish list:

1. Shifters that have numbers. I am drawn to the twist shifters but am not opposed to other ideas. I would think 8 speeds would be sufficient and I would shy away from more as it only makes it more complicated. I gather changing the shifter means changing the derailleur.
2. I love steel frames. Ideas on other bikes or frames?
3. I know it is a potentially controversial topic but what manufacturers are known for what? SRAM, Shimano, Suntour, etc.
4. I had a bike with disc brakes before and liked them. Is it possible to convert to them afterwards?

I would love a resource that explains how bikes go together and what variations in the parts do and why someone who select one over the other. I have always loved bikes and owned a ridiculous number of them in my life but have never really understood the relationship between certain aspects of geometry, what benefit component variations have, etc.

Thanks for your collective wisdom!