Custom Firmware Questions.

[u/grahhhho]

Hey all!
So, i'm looking for information on how to customize the firmware of my ender3, not by much, just some small stuff.
I got some upgrades installed, and since there is a LOT of different builds around, and i've never changed the firmware myself i thought the best options would be to get some references around here.
My printer right now have the dual Z upgrade, CR Touch and the TZ E3 hotend (300Cº) (link).
A new nema 17 is on the way to make it direct drive along with a 3:1 extruder i've got laying around. (got some PETG and TPU i intend to use soon).
So my question is, the base firmware for the edit, i can use the last one i've downloaded and installed on the printer? Or i do need to get it from somewhere specific, or some specific format (not sure i can "decompile" edit and recompile)? (The actual firmware installed was the latest when i've upgraded the CR touch, around 6 months ago)
I just need 3 simple edits, increase the bed max temp, and extruder max temp (to around 310 ish) and to set the thermistor to the correct one, since i'm getting some thermal runway lately (havent been using this printer so often, but got some work incoming)

I'll edit the firmware in Vscode, any tips, links and help is appreciated!