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TBH, it depends on the board/case you have, but from what I read/heard, PC plates generally produces "thocky" sounds and POM plates produces creamier and poppy sound.
I'm trying to wrap my head around the 75% layout - if I want to have Home+End+Delete on the nav column, is it simply a matter of remapping those keys and having keycaps that fit their respective rows? >

Yes, as long the keyboard in question supports remapping in some way. Ideally through the opensource QMK firmware (and thus Via/Remap remapping GUIs), which lets you change any of the keys to anything + has other features. 
Some proprietary remapping software won't let you change where certain keys are/what the Fn layer does etc, and it's hard to tell how good/bad they are in that respect before buying, since they tend to not be super clear about the exact limitations. >

Keyboard for finger / wrist pain:
I have had chronic issues with my dominant hand (work long hours) - specifically with the wrist, the index finger and back of the hand / knuckles around index and middle finger. These issues were partly alleviated with switching to logitech mx vertical mouse but have since returned. 
The issue could be partly caused by my Logitech MX master keyboard (non-mechnaical). As such looking for advice on any mechanical keyboards with light press keys / good for ergonomics. Will mainly use with thinkpad and don't need anything too fancy part from full numpad with portability being an advantage. I have bought a logitech ergo keyboard but would rather stick with the flat setup if possible and doesn't seem to fix the issue with finger pain. 
If anyone else has suffered from similar issues and have recommendations beyond equipment (i.e exercises) I am all ears. >

1) Check out r/ergomechkeyboards and the ergonomics channels in the QMK discord if you haven't, there is quite a lot of stuff out there. 
2) Maybe consider using a trackball instead of a mouse (i don't like most trackballs but the slimblade is pretty enjoyable to use)  
3) Checkout something like an Alice layout with QMK support, such as the Keychron Q8/Q10/V8/V10. Alice layout is a lot nicer than standard keyboard layout while still being flat/not in two separate pieces.  
4) Consider using a smaller layout and using layers instead of moving your hand around the board a lot; reducing movement is generally the best way to reduce the "repetetive stress" that causes RSI.  
5) Definitely try using a smaller layout that allows you to keep the mouse or trackpad closer to your body, each mm you have to "reach"  is added strain.
6) Check out some videos about ergonomic positioning in terms of desk height/wrist positioning and maybe considering making some changes in that respect.
7) Try a separate numpad if you really need one. Or...↓ 
8) Try making use of motion-reducing keyboard functionality in QMK. For example, you can set a layer that turns "UIO/JKL/M,." etc into a numpad that you can use from the home position, perhaps activated by hitting Caps Lock (which you'd have rebound to not actually do "caps lock" in that case). Another example is mouse keys; you could have a layer as well that turns JKLI into an "arrow cluster" that moves the mouse cursor, with like MUO; as mouse buttons/mouse wheel scrolling etc.  
(An Alice layout board gives you extra thumb keys that makes layers easier to use imo)
9) Check out "southpaw" keyboards that have the numpad on the left side at least.
10) Keep your hands warm, as cold muscles/tendons are easier to injure.  
11) I know you said no excercies but some finger extensions with rubber bands should help some.
12) As far as switches/keypresses, the two things would be like "how hard are you colliding your fingers with the board" and "how much force are you having to apply to 'press' a key." Those sound like similar idea maybe, but a switch with a heavy spring (e.g. a Cherry MX Black) requires a good amount of force to activate, but is easy to not "bottom out" on, so your typing is pretty well cushioned by the spring. On the flip side, a very light weight spring might require very little force, but be too easy to smash your fingers into the hard bottom unless you're a very light typer—a potential solution there is getting a keyboard with a soft gasket mount that allows the PCB to bounce a bit (basically imperceptible visually unless really pressing hard, but helps reduce the impact force a lot still).
You can also apply o-rings to the switches (easy to do, just a little tedious) to add some extra cushioning.
That all said, if you just want some boards to buy:  
-Look at the Kinesis Advantage2 if you haven't, even just as an idea of what is out there.  
-Check out the Keychron Q10 for an accessible gasket mount Alice layout board with F keys and macro keys. The V10* is not gasket mount but is cheaper and the same layout.  
-Check out the Keychron Q5 or V5 or K4 Pro* if you absolutely must have an integrated numpad on the right side.
*V10 and K4 Pro are coming out this month. (K series is bluetooth wireless) >

Is there a such thing as a vintage USB mechanical keyboard? 
From what I can tell, most vintage mechanical keyboards use either DIN 5 or PS/2 connectors, and when USB came around, everyone started making memlame keyboards. So is there an exotic USB one I've missed? (Besides the Unicomp Model M's) >

Nice, ergonomic :D Good job maintaining it all these years. >

oil kings or box inks? im looking for a deeper sound >

Ink seems deeper to me. But however both has different fundamental pitches. You may not agree with me, It’s normal >

I'm planning to have a custom artisan knob, does gmmk knobs fit in akko 5075s? >

They should >

Keys are typically measured in units of "u" where "u" is the length of a standard key like A or Q etc., and is always in 0.25 increments so you can always tell the length by lining it up against your letter keys etc. E.g. the Ctrl key there is 1.25u, Tab is 1.5u, Caps is 1.75u & the Shift is 2u. Keys that are 2u and longer have stabilizers, 1.75u and shorter don't. >

What are the sound differences between PC plates vs. POM plates? I'm hearing mixed opinions on which material creates a more deep/thocky sound (my preference)