New iMac or wait for ARM

I suppose no one here knows if and when Apple could switch to ARM.

You can wait, but once this evolution is done, you'll hear of a new one that will change the game. New display ? New memory chips allowing for much higher RAM capacity.

So IMHO, the question is more: do you need it now (it looks like it does) or not ? If yes, I would advise not to opt for a transition machine. If you get a 27", it will last for the next 6 to 8 years. And you'll never care it is an Intel or an ARM. At that time, the resale value will probably not be such an important question.

So, looking at your concerns:

Will the performance of Intel iMacs take a hit when Apple updates macOS for RISC?

New versions of OSX requires usually more resource. But the question is more on RAM and disk space than on pure performance.

Will we be able to build iOS and macOS for RISC on Intel iMacs?

I bet YES on this. Remember when they switched from Motorola to PowerPC and then to Intel: all tools did continue to work.

Will the resale value (should I need it) of an new Intel iMac fall preciipitously if new RISC iMacs appear in a year or two?

See before. If you are to keep it 6 to 8 years as you did with your previous one, that's not an issue.

One last point: keep the old Mac, with XCode installed and OSX 10.14. That may help you one day, to test some code on older systems or to migrate very old code to new version of Swift.