Hi there
I also use wood pellet litter, I buy 15kg bags of a horse stall bedding brand which from memory are roughly 23 litres. Each bag lasts me about 6 weeks for a single litter tray for 2 rabbits and the bit I use for my parrot’s cage. So going by that I use about 4 litres per week which is certainly far less than you use.
You can certainly use the cat litter branded bags if they are cheaper. Here they market wood pellets for wood stoves, horse stall bedding and cat litter. What you use simply depends on what’s cheapest and as long as there are no additives it will all be fine. If you are concerned about the different smell (different brands sometimes use different wood types) you can mix the two types over the course of a week.
I suspect you may either be using too much litter in the tray, or could do with using a tray that has a sieve tray. I use a litter tray with a sieve tray. Basically there’s a normal solid tray underneath which I scatter a small amount of the wood pellets in, then another tray sits inside of top of that and it has small holes in it for the pee to fall through. In the top sieve tray I only put a single layer of pellets, so cover the tray one pellet deep. It really doesn’t need more than that. I clean the tray once a day which include removes all wet litter which has turned to dust and scooping poops. All the pellets from the bottom tray get dumped, and usually in the top tray at least half the pellets remain so then I just top those up. Here’s a step-by-step of my daily litter tray cleaning on the first page of this thread here: https://binkybunny.com/FORUM/tabid/54/aft/152834/Default.aspx 