Here's some food for thought from a longtime 520 guy,
Here's how my ancient desk fits into the modern world.
I'm running 2 interfaces and 16 track tape.
1) One interface is an allen and heath unit set up for stand alone. It has no preamps, just 16 analog in and 16 analog out. I could hook it up the computer as an interface but just use it as a tape machine.
It's hooked up so i can use it as a recorder, and so I can dump 16 tracks of tape to a harddrive which gets proceesed in protools or reaper.
Both can be mixed in the box or on the desk.
2) I have a focusrite interface hooked to the computer into protools.
This as well I can put back into the console for mixing.
3) My trusty MSR 16 tape machine is hooked up to the desk and the interface which is really acting as just an analog to digital converter.
So, as you can see, got lots of options.
The M 520 despite being an rca unbalanced -10db mixer has an 8 channel balanced amp option so you could hook up via xlr cables to an 8 channel interface. You would feed this via the 8 buss faders.
I haven't used this and wonder if the signal coming off the board into the interface would be too hot. Dunno, will have to experiment at some point.
I do know that the -10 db going into an interface works just fine. When it gets into the daw the signal is low but in digital that works out just fine. You can always bring it up with clip gain.
So in closing, I'm using my mixer as part of the "instrument into daw" signal chain. And often I'm using it as part of the mixdown.
But for recording, basically I get the sounds i want with the analog gear then send it into the digital world.
You brought up pfl and solo fuctions.
Solo takes what you have on the channel including any effects you have on the insert and you hear only that channel in its pan position.
Pfl brings that channel up in mono and before the fader.
I like to think if pfl being for tracking and solo for mixing.
One word of caution. Watch out when using headphones!!!!!
Whenever you push pfl or solo and that red light comes on, the orange solo volume controls the level. The green headpine volume does nothing! You can turn it down in a panic and still blow your ears and/or headphones out !
I make it a habit to keep both volume knobs turned down untill I'm actually using them.
You also mentioned cleaning and becoming overwhelmed.
You don't have to take it all apart to clean up the switches and pots. You can spray the deoxit through the top. Then with a shoelace or some string wrapped around the shaft 'massage' the pot.
It's very effective.
Hope some of this helps.
Read your manual, study it and watch some of sweetbeats videos. I think the yt channel is called sweetbeats tech stop.