I can't find anything for 230v other than some evidence that variac dot com made a 240v: 120-240v isolation variable transformer. but its not on their website, just this document.
https://variac.com/staco/PDFCutSheets/VT%20isolated.pdfI could make you one, i've wound a 0-140v variac from 15 gauge wire and it was rather easy, with a little more effort i would have had it done right. i wouldn't be able to do the special silver-cadmium? plating that they put on top of the copper to make the brushes last longer and not corrode. actually, i wonder how long nickel plating would hold up... i wouldn't really be willing to do anything smaller than 15 gauge wire so you'd be looking at a 20 pound transformer..
but for less money and shipping you can just buy yourself a 0-260v variac and a 230v isolation transformer.
this is the only isolated variac i know of that's not 1500 bucks from stacko or variac assuming they even have one in stock
https://www.amazon.com/Variable-Transformer-1000VA-Isolated/dp/B09X153FZWalso a serious question is how much granularity you want. -i have seriously thought about winding a variac which exposes every other turn, meaning a 4 layer inner diameter coil and a 2 layer outer diameter.
a 20 gauge secondary coil (typically used on 5 or 6 amp rated variacs) spanning 180 degrees of the toroid would leave plenty of room for the 230v primary winding to go on the other half of the toroid. -its still going to be a pretty big transformer to fit a traditional single layer 230v winding on half or 2/3rds of the core.