Rookie looking for help w/drivetrain compatibility (Shimano)

[ETIC]

Hi everyone,

Please forgive my ineducation in advance…I have done a few days research, however the more I read the more I am confused. The subject is, drivetrain replacements on an old MTB, specifically, compatibility between base level Shimano gear and different speed systems.

This is my steed - https://archive.trekbikes.com/us/en/2009/trek/3700#/us/en/2009/trek/3700/details

Before anyone says ‘bin it’, I know, but I’m very attached to it now.

I currently have the stock crankset, i.e. SR Suntour XCC-T208 48/38/28 170MM - https://www.srsuntour.us/products/nex-t208?variant=27528530371

(fully aware it is total rubbish).

Back freewheel replaced last year for this - https://www.decathlon.co.uk/shimano-tz21-7-speed-freewheel-bike-cassette-14x28-id_8063280.html

Back derailleur is stock 7-speed Acera, shifters are stock indexed 7speed Acera

Chain is aftermarket 5-8speed, replaced with the freewheel last year - https://www.decathlon.co.uk/kmc-z8-5-8-speed-bike-chain-id_699035.html

As you can probably tell, I am a low earner and therefore quite a cheap person. I’d like to replace my crankset/BB (both are making all sorts of racket and the chain rings are worn down), maybe with something ‘nicer’ second hand from ebay (or is this very risky for low end products?)

Questions:

- how interchangeable are Shimano series products? Acera / Altus / etc? I've come across the Shimano chart but it is thoroughly confusing

- how interchangeable are speeds between front and back, i.e., could I use a 9-speed crankset with my existing 7-speed system?

- would I spend lots of time trying to find a crankset compatible with my rear derailleur/freewheel, or should I bin the rear derailleur as well and change it?

- if the above true, I assume since I have a 7-speed indexed shifter, I would need to either replace the derailleur with a 7-speed, or, replace the shifter and 7 cog freewheel too in the case of say, an 8 or 9 speed shifter/derailleur?

[Painkiller]

If that is all you need is a crank and matching Bottom bracket you will be fine with most any 7,8,9 spd and still be able to use your chain, derailleurs and such.