When does autumn syrup become a liability — nights under 10°C, do I pull the feeders and switch to fondant now?

First year in Yorkshire with two colonies and I think I've hit the limit on syrup. Started 2:1 (by weight, learned that lesson on this forum) on the 12th of September and both hives took about eight litres each in the first fortnight, really quickly. This week the uptake has basically stopped — there's still three litres sitting in each rapid feeder and it hasn't moved in five days. Nights have been 6-7°C and the days are struggling to 13. Someone at the association said that at this point syrup becomes a liability because the bees can't dry it down and I'll end up with unripened syrup in the comb that ferments over winter and gives them dysentery. Now I'm worried I've already done that. So: do I pull the feeders now? Is the syrup that's already in the comb but not capped a problem? And is this the moment to switch to fondant instead — or is fondant only for emergencies? Hefting the hives feels "heavy-ish" but I have no idea what heavy enough feels like.