Nuggets for low calcium diet

Hi everyone, I have read a lot of topic but my doubts are always here and therefore I would like to ask you.
I have two piggies, both have bladder problems, one had a bladder stone and from that moment I changed their diet.
I was giving to them 'Bunny nature guine pig dream basic', which is low in calcium but is no longer available on zooplus and elsewhere are really expensive.

I had thought of giving them 'Science selective grain free', but there are those who say that they are good for a low-calcium diet and those who say no.
Do you think I can give them these nuggets for their diet?
Or if you know any other types just tell me :)

I think most of our UK people use Burgess.

Not a pellet, but still food. Not sure if it is too expensive.

https://www.thehayexperts.co.uk/fiba...-pig-350g.html