I appear to have the same issue after upgrading to 3.4 on a reasonably small model (51mb). Enscape hangs at '99% export materials' for 30+ mins. Current drivers, fresh install, nothing helped. After troubleshooting the model for a while I found that deleting proxies made Enscape load just like it would before. I copied those models back into my main file and haven't had any issues since. Not being able to use proxies is a kind of a bummer but it's what works now... Hope this helps.