I also have mine on Pentosan which did have a noticeable difference for him so that's great. You could possibly talk to your vet about this, it's significantly cheaper than Beransa (110AUD/month) and for my dog it's biannual with a booster shot in between

I actually (anecdotally) heard a similar issue with Pentosan that it stops working after 12-24 months due to the immune system so you have to "time it" at an appropriate time in a dog's life. To be blunt, by appropriate I mean you'd want to start using it 12-24 before the dog is expected to pass and not at the first sign of osteoarthritis.

https://en.wikipedia.org/wiki/Pentosan_polysulfate#Dogs