Sorry if this is the wrong area...

I have a mongoose xr-75, got it used a few years ago.
It's in fully working condition, but it is a bit worn out n rusty n weathered.

I want to start riding it on the road a bit but mainly these flat gravel trails my neighborhood has... it is kinda a fine gravel, been a while since i've been on it but I think the rocks where about dime sized.

low spec, new and pretty cheap parts.

the things that I would like to now/eventually get up to "low spec"

the seatpost is far too short for my long legs - $20
all the cables could be replaced - $???
some more onroady tires? 26x1.95 -$40?? pair
I cant quite remember if it was the cassette or the crankset that I saw some wear on, but both look to be in the $20 area
rims are kinda wobbly, idk if their cheap or not, to replace :/
derailers need tuning.

The brands that I see are giant for the seatpost and crankset
shimano for the cassette.
Giant K935 Comfort ATB Tire is a tire tread that I kinda liked, but didn't know how it would really perform for what I want.

are these parts worth it? how compatible are bicycle parts?
my priorities are:
seatpost (I found out the diameter, but need to check how much longer I need)
cables/derailer tuning, if its cheap enough.
cassette/crankset
tires, these will probably outlast the gears, but one is dryrotted, also wondered if I could put a slimmer tire in the rim 26x1.95 is what it has now, but when on road you can really feel the drag.
rims, a bit wobbly but not too much concern.

but then again... I can go buy a new bike for about $100
but I'd still need the seatpost, and tires, as I do like to do speed runs on the road.

Ask me anything, I would like to get these questions answered as best as possible.

Thanks,
Awaqa909