You sound like such a great owner to be making these changes
I believe Erinsanimals has a video on making wire or mesh wheels safer:
https://www.youtube.com/watch?v=KWbGPfVw_3c
However, would it not be possible to order some plastic ones online? Amazon, for example - or perhaps Zooplus. If you ordered your cages from there you would get free delivery (as long as you're in Europe) so you could get some wheels at the same time.
With their diet, I'm far from an expert, but there are lots of threads in the feeding section that help to guide you. You sound like you're aware of diabetes which is great. What mix are they on currently?