Looks like it would be easier to replace the drive knob with a smaller value logarithmic pot - that’ll also reduce the gain, and will give more control over lower drive settings.
Any of these:
12AX7 - gain of 100
12AT7 - gain of 60
12AU7 - gain of 20
because they’re preamp valves they don’t need biasing, and can literally be unplugged from their socket and replaced; they only fit in one way, so knock yourself out!
Look down the neck from the bridge end - if it t looks straight then it’s a case of lowering the saddles on the bridge, if the neck looks bowed then it’s going to need the truss rod tightening.
could you post some pictures?
Bear in mind that the expression pedal hasn’t got any audio signal running through it, so there’s no signal degradation in whatever you use - whether it costs £17 or £170, it’s just a potentiometer, some connecting wire, and a pedal with a hinge.
I’m not a fan. The SX I had years ago had the older headstock. I have a maple jazz neck on one of my bitsa basses at the moment, and I’ve toyed with making up some veneers to cover it up!
I’ve got a Vintage (brand, not age) SG that plays beautifully. I’ve retro fitted a Bigsby style trem, and put coil-tappable humbuckers with push/pull pots to go between humbuckers and single coils.
At its default setting it’s wired as a pair of single coils, and I find that I rarely bother with hunbuckers- it sounds great like that.
Upgrade your snare to one with a mesh head - arguably that’s the one you’ll be using most often, than think about your kick.
a better quality sound module will help too, but I think the feel of the snare is the most important thing.
One caveat to my vox amplug useage - I use the ‘clean’ version, intended for guitar. It doesn’t have the metronome or bass amp sims (but the fender bassman setting is pretty good), but I often like to practice without any amp sims, just to concentrate on my technique. It’s also really useful for playing guitar through - so there’s that.