Vinyl Newbie - So Many Questions!

New to these forums and new to vinyl, but longtime audiophile, so I feel comfortable saying that I know what I’m hearing but can’t figure out why. 

It would probably help if I summarize my system to aid in troubleshooting:

Marantz TT 15S1

iFi Zen phono preamp

Naim Uniti Atom as streamer/preamp/dac

Conrad Johnson CAV-45 S2

Klipsch Cornwall IV

I’ll start with the easy one

1.  I followed the setup instructions for the Marantz tt meticulously. Every time I start the motor, the belt squeaks and slips on the motor pulley before getting up to speed. It’s annoying and really cheapens the experience. Is this normal?

2.  The sound quality ranges from very bad to very good, largely dependent on the record. I don’t have a large collection as I just started with vinyl, but many of these pressings sound absolutely terrible. Many sound distorted and compressed. Many have tons of clicks and pops. I double checked the settings on my phono preamp which is really just moving magnet. There aren’t a lot of settings. One for MM and three for MC. I connected the phono preamp directly to the Conrad Johnson instead of the analog input on the Naim and while it does sound “better”, it’s mostly the same. 
 

I don’t have another cartridge or phono preamp to swap out and try to isolate the issue. Could it simply be that the phono preamp is defective?  It seems that if the cartridge was defective it would just all sound bad.  Could it be that my system is revealing enough that it’s showing the limitation of the iFI?  I just think it would sound “good” all the time, but never really “great”. Could it be that all of these pressings are so incredibly crappy that I’m hearing just how bad they are?

The couple of “mainstream” records do sound pretty good. Namely Gregory Porter, Adele, Gary Clark Jr, Miles Davis, John Coltrane. But even a couple of those have inconsistently cut holes for the spindle (some of them bind), significant warps, and some off-center labels. The independent stuff is all over the map. A couple of the John Prine pressings sound very good, but have soooo many clicks and pops it’s ridiculous - even after cleaning. And one of them wouldn’t even go on the spindle because the hole was so small. 
 

Anyway, I’m just perplexed and pretty annoyed. I’ve debated buying another turntable (with cartridge) and phono preamp just to see if there’s any change.  Any suggestions?  Is that my best course of action - to just buy another preamp and cartridge to troubleshoot?