I would take them to a commercial laundry…just make sure to NOT dry them. You must replace them while they are damp.

i have no experience washing slip covers. maybe a machine with no agitator would be roomy enough. or commercial front loader at a laundromat.

A used SQ circa 2017 or earlier with analog controls would work. You can hold the reset button to fill the tub to above the top row of holes. You would still want to be actively involved with the process-making certain there is indeed enough water to assure the covers are being cleaned without being stressed by the washing action. I would not recommend buying any machine just to wash one item that only occasionally needs to be washed. To avoid going to a laundromat, you can always set aside some time to wash the covers in a bathtub, rinse them thoroughly and then spin them in your machine.

I also have an IKEA white slipcovered couch that I've had about 1.5 years. Initially I planned on taking the covers to a laundromat so I could use a large capacity washer. However, I just never found the time to do it.

I eventually washed it in 2 separate loads in my home washer. I have a Maytag 835, it's a few years old model.

The slipcover came out fine and washed easily. I used a deep fill option, but if you have a soak option I'd use that just so I could make sure it gets really soaked and washed all over.

I washed the base cover first and then the cushion covers second. It worked out just fine and knowing this I will wash it more often now, which should also help it stay cleaner.

I also spray some Bayes upholstery protectant on it.