How often do you clean them and replace them?
Not often enough

We usually wait until there are particulates in the water. Hate to admit that sometimes 3 months goes by. If I were organized I'd have a schedule and pull out and replace the cartridges with clean ones/dump the dirty cartridges in bleach to clean...too lazy I guess. Hope to do better on the big system. Sheer laziness on my part not to bleach out the cartridges....just got 8 more cartridges last week. For the last two years I've thought that my new tank would be set up soon and I wouldn't need the 25s...but it hasn't happened. I definitely will be bleaching the 100s. I bought a cartridge cleaner that you connect a hose to....don't bother getting one. The cartridges will get so jam packed with debris/poop, that I don't even think a pressure washer could clean all the junk out. The person who sold us the RTLs and the Hammerhead pump said to bleach the cartridges.
How often you change the cartridges will depend on your stocking. These spa filters sure are a painless way to go for great mechanical filtration. I love not having to vacuum the 300 gallon tank. There's no way I'd want to vacuum the big tank we'll be setting up.
Poop suckers are a great way to drain the tank. We have a pipe going under the house and out. All we have to do is open a valve and the water drains outside. Because the poop suckers are just off the floor, we could literally drain the tank dry (so we have to pay attention) The water drains by gravity flow very quickly. Since you have an automatic water change system, you probably don't have a need to ever drain your tank. It's just an very easy thing to do with the closed loop mechanical system.
The other thing we'll be doing different on our big system is that our mechanical filtration will be very prominent. The pipes will be the only decoration in the tank. We are literally going to have a forest of poopsuckers/uptakes/returns. Here's one of the smaller poop suckers which still needs to be painted.
When it comes to spa filters, they have been used by fishkeepers for years. It just isn't common practice for smaller tanks that can get by with a canister or HOB filter. I think it's one of the best kept secrets in fishkeeping. I would never consider not using RTL filters. Our two little RTL-25s have provided us with extreme mechanical filtration. I experienced daily vacuuming of that 55 gallon tank that all our monsters came in. I don't plan to EVER use a python or do any other kind of vacuuming of debris again. If anyone is gravel vacuuming out a 300 gallon tank, I feel very sorry for them. Around here, if it's not extremely low maintenance, it's not going to fly. We love our fish, but hate any work associated with them. It just ain't gonna happen.