Since your pH crashed again, keep watching closely for a few days to make sure you don't see ammonia show back up, if it doesn't you're probably in the clear as far as that.
But as far as your water kH goes I think you're not going to be able to get away from monitoring that daily, or at least every couple of days, to prevent another crash. You'll want to keep it around 80ppm or about 4.5* to be safe.
Below 40 for the fishes health.
You'd be better off keeping it lower since you have soft water and you're trying to maintain your kH and pH, and high nitrates will deplete buffering. If you maintain it around 40 as soon as you slack off it (which is pretty much unpreventable, IME, since life tends to get in the way of maintenance sometimes), then it will rise higher and possibly cause issues again.
I clean out the sumps and change/clean the polishing media at least monthly (well, optimally). The higher your stocking levels the more often you will have to do this. Since you seem to have a fairly heavily stocked tank then you want to do it as often as possible. Going 8 months without maintaining your filtration is definitely WAY too long for any decently stocked tank.
Dirty media and sump will definitely lead to high nitrates from the detritus decaying. You should also be siphoning at least half the substrate weekly.
For me I find barebottom MUCH easier to maintain with a heavily stocked tank since a lot of detritus will fall to the bottom (especially with something like arowanas, since their waste is pretty big) for me to siphon out when I see it. Barebottom is not as attractive, but it is for sure easier IMO. Some arowanas, though, may take awhile to get used to a barebottom tank if they're used to substrate. Mine was raised in barebottom tanks so he was used to it. In fact when I added substrate later on he found it a little suspicious.
But as far as your water kH goes I think you're not going to be able to get away from monitoring that daily, or at least every couple of days, to prevent another crash. You'll want to keep it around 80ppm or about 4.5* to be safe.
Below 40 for the fishes health.
You'd be better off keeping it lower since you have soft water and you're trying to maintain your kH and pH, and high nitrates will deplete buffering. If you maintain it around 40 as soon as you slack off it (which is pretty much unpreventable, IME, since life tends to get in the way of maintenance sometimes), then it will rise higher and possibly cause issues again.
I clean out the sumps and change/clean the polishing media at least monthly (well, optimally). The higher your stocking levels the more often you will have to do this. Since you seem to have a fairly heavily stocked tank then you want to do it as often as possible. Going 8 months without maintaining your filtration is definitely WAY too long for any decently stocked tank.
Dirty media and sump will definitely lead to high nitrates from the detritus decaying. You should also be siphoning at least half the substrate weekly.
For me I find barebottom MUCH easier to maintain with a heavily stocked tank since a lot of detritus will fall to the bottom (especially with something like arowanas, since their waste is pretty big) for me to siphon out when I see it. Barebottom is not as attractive, but it is for sure easier IMO. Some arowanas, though, may take awhile to get used to a barebottom tank if they're used to substrate. Mine was raised in barebottom tanks so he was used to it. In fact when I added substrate later on he found it a little suspicious.
I'll try to continue to monitor my water parameters. My tank is bare buttom, I've added the crushed corals in baskets into my sump tank. 
