How to stop erythritol crystallisation?
This is simply the nature of erythritol. In my testing, I wasn't able to create any kind of viscous, syrup-like consistency by mixing it with water and heating it to reduce; rather, it would always form a hard crystalline structure. This most likely has to do with the fact that, unlike regular sugar (sucrose), erythritol is not hygroscopic at all. (Things that are hygroscopic like to hold onto moisture for dear life; erythritol simply lets go of the moisture and you end up with a hard crystal).
If you are in the US, I'd recommend looking into allulose. It's extremely hygroscopic and I've successfully used it to create a syrup-like consistency. (On a scale from 1 to 10 — 1 being the least hygroscopic and 10 being the most — if erythritol is about a 3, allulose would be a 9 or 10). Be careful, however, as it burns much more easily than regular sugar does. Some downsides of allulose are that it's more expensive than erythritol and it isn't approved in the EU.
I have less experience with xylitol, but I believe it is also hygroscopic enough to be able to form a syrup, but I'm not positive. (Just a note in case anyone wasn't aware: xylitol can be fatal to dogs and cats). I did one test with xylitol but had problems with it crystalizing, but after reading the guidelines at https://willamettetransplant.com/xylitol-simple-syrup/, I realized I likely wasn't careful enough in creating the syrup. It seems any undissolved crystals or stray crystals that come in contact with the syrup can serve as seed crystals to hasten re-crystalization.