If you're prepared to take drastic action as you suggest then consider using Thunderbird and importing your outlook email in to it.
Take an image with your backup software first though, just in case....
I have an ASUS ROG Crosshair VIII Hero (WI-FI) which has 8 sata ports. It is the X570 chip.
It is a solid board and using 32 GB RAM GSkill 3600 MHz and a Geforce 950 video card.
It only has 2 M.2 slots though.
Typical ROG reliability, which ever board you choose the 2 stticks of ram is the way...
My windows installs have always steadily increased in size and i put that down to log files getting bigger and just mysteriuos stuff.
I've never given it much thought especially since i give the partition a generous size and let windows do its thing.
My browser can sometimes end up with a large cache so check that.
Windows Update download maybe?
There is a free program called WinDirStat which can analyse your drive and give a graphical representation of the contents....
https://windirstat.net/
I use several truly free programs:
Karen's Replicator - or anything Karen created before she unfortunately passed away
Cobian Reflector
XnView
KeepassXC
Bitwarden
Veracrypt
Github or Foss are good resources when looking for free/open source software
I use Home Assistant
I grew tired of having similar problems so i opted for a program called iMazing.
It's very good and there is no bloated hard to use iTunes.
The licence is per device not per computer or user so you can install it on any number of computers.
They have excellent support services.
As cerberus says, do them individually as this gives you more flexibility.
I have C full weekly with differential daily
D for documents weekly with differential daily
With the above i can restore C and not have to worry about losing emails etc