I have one & I really like it, but from what you've written I really don't think it would solve your problem. Sliders make the surface smoother to move the quilt on (and yes, there are cheaper alternatives), but they won't change the amount of pressure from the foot or the quilt getting "smashed" through the needle plate.
Instead of a slider, here's what I'd try first (if you haven't already):
- Stitch length is "0"
- Single hole needle plate is installed (this will keep fabric from getting smashed down through the hole -- it is absolutely essential; you can buy it 3rd party for most machines)
- Top tension generally is pretty loose (usually somewhere between 1-3, depending on machine)
- Reverse pattern foot is installed (that's your basic presser foot to sew everything)
- If you're using a high loft poly or Warm & Plush cotton batting, maybe try a low-medium loft batting instead and see how that goes (Warm & Natural, Poly-Fil, Hobb's wool, any Quilter's Dream batts)
- There are a few methods for moving the quilt to FMQ. You could try just barely touching it with your fingertips to move it around, in which case I'd recommend Machinger's gloves or something similar with grippy tips; you can move hands a bit away from the needle & grab a fistful of fabric like it's a steering wheel to pull it different directions (typically, fabric is grabbed from underside); you could try one of those hoops or Bat Scooters where your hands are raised. Your forearms should be brushing the tabletop, parallel to the floor. Sometimes slowing down the machine speed helps to get into a smooth, relaxing rhythm.
If you were saying that your stitches weren't even, I'd suggest a Slider. If you were saying that your muscles are sore from moving the quilt, I'd have ergonomic suggestions for you (sink machine in table, body mechanics, suspension system). But as I understand it, it sounds more like an issue with your machine (either different set-up is needed, or perhaps it needs maintenance/adjusting). I work with high loft batting (Warm & Plush) plus I have layers of needle turn applique that I quilt through. I do need to lower the pressure on my presser foot because of the high loft batting & extra layer(s) of fabric, but the only difference between FMQ with Slider & FMQ without Slider is that my stitches are more uniform in size with the Slider because the friction is reduced. I can definitely move my quilt around without issue whether I'm using a Slider or not.
If none of these suggestions work, you could try a Teflon baking sheet & cut your own hole in it -- but again, it really doesn't sound like it'd solve your problem. If you do use the Slider, it will cover your feed dogs, so you would want to put them down.