I'm not with Laminar Research, just a fellow simmer in the UK, but as you are probably well aware, there could be a number of possible reasons why you may be seeing these 'beachballs' on your iMac, usually because the hardware can't handle the software tasks in hand:-
Whilst I recognise that you have been running X-Plane 11 successfully before (not sure which iteration of X-Plane 11 you had?), it is a very system hungry application so it would be worth double checking your full system specification against Laminar's 'minimum' and 'recommended' requirements:- https://www.x-plane.com/kb/x-plane-11-system-requirements/
I have looked at the specifications of most 2015 iMac models and they all appear to have just 2GB of GDDR5 video memory, which is borderline when it comes to running X-Plane 11.
I'm not sure which CPU you have, but if it's less than 3GHz, this again is borderline! As far as RAM is concerned @ 24GB, this appears to be more than enough and I also assume that your HDD will more than up to the task @ possibly 1TB?
You may wish to try benchmarking your PC online at:- https://cpu.userbenchmark.com/Software. It's a free download that tests all of your PC's hardware and provides an online report for you to read, as per the screen prints below.
Sorry I can't be of more help :-(