I've never even heard of such a product because I've never had this problem. I have the Maytag Commercial machine and it had a solid extra rinse feature. It drains into one of the basins in my laundry sink and I've watched it in action - the water from the extra rinse cycle comes out very clear.

I've heard that the reason that your laundry gets gummed up is from using too much detergent (I used to be terrible about this). You can probably try using less detergent or occasionally running the towels through the machine without detergent. Of course the manufacturer is going to tell you to buy another product of theirs rather than tell you to try using less of their product...

I use only a little detergent already. If I run a dedicated rinse I don't see much in terms of suds. So I'm thinking there is something else that's preventing the detergent from being removed completely.

The detergent you're using is soap. It takes hot rinses to remove it from your clothes. And it will build up in your clothes and linens making them less absorbent. I would try a different detergent, not a plant based one, at least on towels. Powder it better. The laundry rinse basically acts the same as white vinegar. Which will help removing residue from detergents and soaps.

My water is already quite soft, but it looks like a warm rinse is a must and that my machine cannot do. There was a time when I would run an express warm wash, but I didn't think that made a difference to how well it was working with respect to residues. I have never tried the Laundry Rinse product but based on what beaglenc says, it may help. Sonett also does make a water softener, so I get it all comes together -- water softener + residue forming soap + residue removing laundry rinse. :)

https://www.sonett.eu/products/waschen/softener/?L=1

https://www.sonett.eu/products/waschen/laundry-liquid-lavender/?no_cache=1&L=1

https://www.sonett.eu/products/waschen/laundry-rinse/?L=1

Does anyone have an idea how this product compares to last rinse fabric softeners sold by many companies?

This is not a softener.

I am assuming you are not using fabric softener or dryer sheets. They can make towels less absorbent. I would try washing your towels on hot in a different detergent, not soap. Then a few good hot or warm rinses. I would spin at a lower rpm and then dry in the dryer on low or medium heat. If the detergent gets all the fat out from the soap the towels should be more absorbent. They might not be as soft, but should be more absorbent. And fro clarification, they are 100% cotton?

I would not wash towels on wrinkle free. I would use extra white and hot or very warm.

I use wrinkle free with the hot water setting. I use that because it uses more water so I thought it would be better at getting the detergent out. In the past, I used the sanitize setting with sanitize temperature or hot temperature and the results were the same. If at all, I often had to run additional rinse cycles beyond what came with the sensitive option.

No fabric softener or dryer sheets.