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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.
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.