Bit of a devil's advocate here
This is a tough one. Finding someone to work on these vehicles is not that easy, finding folks who can competently and acceptably work on them to ones' standard is much, much harder. Add in that the shop is willing to let you source and pay directly for some of the project and this may be a relationship worth saving. That said, I would be concerned and very irritated by the missed deadlines.
I would see them again and very clearly state your expectations; a complete and honest status of the car as it sits now, open issues and when the car will be finished. If you don't get the warm and fuzzies after this talk...then by all means move on quickly.
I don't use shops that often, but I am lucky, no make that blessed, to have a friend who owns a renowned Rover independent shop that works on most oddball stuff and am near a very good Porsche indy. Even then, I've had missed deadlines and the occasional charge that we have discussions over. I will gladly deal with the occasional hiccup to have folks work on my oddball (and that is key) cars who I trust and do capable work.
Also, sometimes s%&t just happens.... Just had our airplane down for 3 mos waiting on a part. After it was finally done, it is still not flying right, safe, but the rudder trim (one of the items we had noted) was off. Back and forth again, few test flights and another month. Mechanic is in contact w/ the manufacturer, drawings sent, etc. I then get a call that basically said 'we reassembled something wrong, it is a complicated component, this may have been wrong since you owned it but I cannot say and I'm not charging you.... He, the mechanic I've used for 19 years,
had ~15hrs into the trim job. He gets a very nice holiday bottle or something.