So I listened to some of the stuff and overall I'd say it sounds pretty nice! The thing with straight-up jazz is that you really want to go light on the processing, since the musicians generally don't like the integrity of their sound tinkered with too much. And that "natural" sound is kinda the vibe anyways (as apposed to rock/pop "better-than-real" sound).
I'm not familiar with micing (or really amplifying for that matter) upright, but I'd assume it would be best to close-mic the body, then get a (DI?) off the pickups.... if that's how it works? Maybe a room mic would be nice too? Could help with blending those nice overtones with the plucking.
Also, that's a wonderfully warm alto sound! I'm generally more partial to bright and aggressive alto, but her sound is warm AND fat! I usually associate warmer alto tone with having a thinner sound, so it's very nice! That said, if you listen to around the middle register (starting with their middle D up to around G, sometimes A depending on the player) those notes can be very resonant, especially on Alto. I'll usually fish around that frequency range a bit to scoop a little out so they don't poke out more than the rest of the range. I'll usually not do it unless I start hearing it though, in which case I'll slap an EQ at the top of the FX chain.
I thought the drums sounded good! For jazz you actually don't want a super present kick sound from the beater, it's more about that fat, resonant "tail" that comes off it. Honestly I can't say I'm the biggest fan of that sound, so I personally shoot for somewhere in the middle (since the modern sounds kinda sounds a little too... fake-ish? Well kicks I've heard when playing in bands don't sound quite so tight anyways, though I don't do much rock/pop!).
Mixing in samples can be a hot-button issue with some people, though I haven't ever done it myself. That may be something between the engineer and musician though, and I have heard some musicians form strong opinions about it. I'm impartial, as long as it sounds good, right?
Hopefully I can get some stuff published soon. We're playing at our local History Museum in December (they're doing a video game exhibit) that will be streamed, and I'll be able to grab multitracks off it which is awesome, so I should be having some fun with that later this year!
Keep up the good work man, sounds great to my ear so far!