Appreciate the feedback.
I watched the sailrite videos too, but they were too much. I ended up referring to this video:
https://www.youtube.com/watch?v=Gw_40jnjZBE. In this video the cushion is small enough that three sides of the cushion are made with two pieces of boxing, but most of mine were made with four.
I have a hard time believing that ANY sewing machine couldn't do box cushions, even in Sunbrella. I use this machine, available at Costco and Amazon for not too much:

It has no problem with hemmed sunbrella and a zipper, or two layers of webbing and two layers of sunbrella. It sometimes needs a little helping push on the wheel, and I've broken four needles so far, but overall it's doing a fine job.
A few things that I learned that weren't mentioned in the video:
1) When sewing the zipper closed ( the seam that you rip out after ), sew the first few inches with a short stitch, but when you are sewing the part that will be ripped, increase the stitch length to maximum. It's really hard to rip out the short stitches and you risk damaging the fabric.
2) Hot-knife cutting is waaaaay better than scissor cutting, ESPECIALLY on sunbrella fabric.
3) With the heavier UV marine threads, the bobbin-winding process is really important. You have to get it tight, because the thread slips on itself and doesn't hold the "wind" very well. Near the end of the bobbin the thread can start to jam, really frustrating, until I figured out the cause. When that happens, I just take out the bobbin and re-fill it, wound tight.
4) When you have finished cutting the boxing and are ready to sew them together, you start with the zipper piece and one of the side pieces. It is REALLY important to choose the proper side piece to start with (the left one or the right one). If you choose the correct one, the cushion top piece will be on your left later on when you are attaching it to the boxing, so it doesn't have to pass under the machine arm. If you choose the incorrect one, when you come to attach the top piece everything will be on your RIGHT when sewing, and have to pass under the sewing machine arm. It's really hard to think it forward and start attaching the boxing pieces in the correct order.
I actually enjoyed this project a lot. So much that I'm thinking about the cockpit cushions next. For that, I have to decide on the foam and fabric. I've read the archives here, and I know we have a professional designer on this forum who recommends sunbrella and closed-cell foam, but I've seen a couple of recommendations that suggest an alternate route. Sailrite says:
https://www.sailrite.com/outdoor-cushio ... oam-thread and now I'm leaning towards this foam
https://www.jtsoutdoorfabrics.com/Outdo ... 19050.html with a phifertex mesh
https://www.jtsoutdoorfabrics.com/Phife ... 12371.html as the bottom panel and Sunbrella outdoor furniture fabric as the top and sides:
https://www.jtsoutdoorfabrics.com/Sunbr ... _3263.html. YKK zippers,
https://www.jtsoutdoorfabrics.com/5-YKK ... 17265.html. The idea is that everything is mold/mildew resistant, and more or less open-bottom for the cushions. The foam won't wick and the bottom mesh should allow everything to air out. The alternative is just seal everything up with Sunbrella coated with 303, but with that method any water that does get in will have a really hard time getting back out, too.