Ok thanks for your input and encouragement Nbt and Anayar.
Good news! After spending the last 2 or 3 hours systematically adding and removing various entities and testing a variety of different combinations of settings, I finally found out what the problem is!
Quote: "Maybe your system just cannot cope with large maps with the extra overhead of 1.17+ ??"
Quote: "Bruce... Can you post your specs. here? I'm sure it would help us out finding a solution..."
You were both close to the mark! It turns out that my PC can't handle full light mapping!
I should have realised this sooner as the clues were all there. Whenever I had the 'Waiting for Threads to Finish' problem, the test build would stop right at the end of the mapping light objects stage!
Quote: "I advice you not to Get discouraged and stop working on your project... No matter how many problems you come across you'll get a real satisfaction knowing you overcame the problems and finished the game"
You're absolutely spot on Anayar, now that I have finally solved my problem, I feel quite pleased with myself! I can now get on with developing my game!
However, I'm not going to risk re-updating to 1.18b1, I can't help feeling I'd be tempting fate! I'll wait until the official, stable release of 1.18 before updating again.
Anyway, thanks guys, your help and encouragement mean a lot to me!
(Edit) After Anayar's comment about flaming FPSC (he was absolutely correct in his comment), I edited my previous post as I didn't want my WIP to turn into flame bait!
I've also gone back through some of my older posts and realised what an idiot I must have sounded, moaning about FPSC every time I hit a snag! I have now deleted the parts of those posts where I was unfairly blaming FPSC.
I game therefore I am.