I have never used the Fluval brand diffuser. I actually have only owned one ceramic one that is in tank (use a GLA Atomic and I believe it's made of something else). That said, after many years of use, I haven't seen any difference. It has degrade very slightly as I can see a tiny bit of white stuff after I bleach. Still functions the same as the day I got it, probably 6 years later, with regular cleaning.
As per your bubble count, it will go down significantly when changing from a reactor to a diffuser. The diffuser needs more pressure to get going as you have to force the gas through. When I used DIY CO2, I actually couldn't get my setup to work with certain recipes that were supposed to last longer and be more consistent. I believe the best diffusers for DIY CO2 are the Ebay ones from Hong Kong as they don't need as much pressure to work. The better diffusers (smaller bubbles) need more pressure to work so for DIY, you actually want a not so great one.
You can try a chopstick or BBQ skewer to see if you get more CO2 in for a cheap solution to test. They both work pretty well but usually need less pressure than a ceramic disk type. Just stick it in your CO2 line about 1/4 in or so and cut the rest off. If it works better, you can just use that instead if you want. Or go buy a cheap one or two and use that while it ships.