Ice Cap Zone and Starlight Zone are pretty nice, their slopes are smooth and they are pretty as well. However I still have some issues with both.
The signposting on the first turn is very deceptive, I found myself bonking the left fence that cuts you off from the green road, since that road is all you see at the start and the signs point you in that direction. The row of red springs in the first cut(?) is too narrow, you can go through them if you're going fast enough. I'd recommend making them a fair bit thicker. I didn't spot the cut for the first few laps, its was hard for me to notice. Also you can take it for free, there is no offroad on the building, making me unsure if this is a cut or a free path. I would consider adding offroad since this would invalidate the road next to it if its supposed to be free.
The first turn after the first loop is hard to see coming due to the upward slope blocking your view, I'd recommend making the sign clearer to see and adding a fence to the outside, and maybe moving the slope closer to the loop to give you more time to read it.
The item set before the next loop is hard to see coming, I'd move it to the top of the slope it's on so you can see it from farther away, or removing it entirely since there is an item set directly before and after it making item cycling/power plays a real concern.
The cut after the last loop is very strong (it saves 3+ seconds) and is not far from the finish line. Between that and all the item sets, this means that it'd be very easy to use it to sandbag for wins on the map without being open for retaliation. You might want to close off this cut on lap three and possibly nerf it slightly so that the map doesn't promote toxic play. Also I had a difficult time spotting it, I only noticed it when going backwards to test something. You may also want to move the item set backwards so that players pulling from it can roll their item before the cut, or forwards so that you can't pull the cut and the item simultaniously. This sort of depends on the item set before it.
Other than that the turns feel good, I like storing drifts off the loops, and outside of the mentioned readability issues it was pretty easy to read. Despite all the complaints I think this map is my favorite out of the bunch, but I am very worried for it's balance in netgames. Regardless the map is a vibe and is what I feel like Starlight Zone should play like in kart.
Ice Cap Zone
There is a cut that you may not have been aware off during the first split path, you can use a sneaker to launch yourself onto the lower path. Its a fair bit faster than committing to either path. I don't think this cut is harmful to the map but I figured you might as well be aware of it.
One of the final cuts is kind of awkward, due to the angle you enter it at and the speed of your sneaker sending you into a wall. This means the most optimal way to take the cuts become strange and unintuitive ways, such as mowing the cut and using your sneaker after, or launching yourself off the inside slope into the final two turns. The latter option makes the cut both stronger than usual and also difficult to punish. It still only saves around 2 seconds, but its worth considering how close to the finish line it places you.
I think Ice Cap is pretty good, basically all of my problems with it that I picked up on were with that cut being a little funky. Its a pretty map and the slopes are extra nice with boostlib.
Mystic Cave Zone
I am pleased to see some of the stuff I've talked about has been improved, but there are still some things that are worth mentioning again.
Item Odds on the first split path are indeed messed up a bit, testing with multiple players its possible to get a power item or something similarly powerful depending on player count by taking the opposite path from first. Remember your item odds are based on your distance to first place roughly, not your distance to the next checkpoint. You could probably fix this by moving the first item set either earlier or later in the map, so items are rolled either before people separate or after they come back together. I would personally recommend the latter more to give people a little bit of time to spread out naturally before slugging eachother. This might seem small but it can mean a lot when second gets a star or triple shoe almost directly behind you.
The cycle hazard after this section has moments where both sides are blocked at once, I think that requiring someone to slam their brakes isn't preferable personally and you may want to adjust the timing so that one side is always open at any point in time. You don't want players feeling like they just got unlucky with the cycle timing.