Update: I started the recording again from scratch because when I started the original recording above, I'd tried to use Rewire in Reaper for Macs right at the start - I couldn't get it to work properly (my first try) and when I tried to get rid of it, it wouldn't go away completely - it kept telling me that files were missing but the audio was being kept. I thought that might have been the problem, but that wasn't it, because when I redid the whole thing and rendered it again it still had clicks and pops.
This second time, I increased the Render Ahead size to 400 (from default 200) in Reaper preferences, and watched the render in Apps>utilities>activity monitor for CPU useage. Before rendering, nothing clipped in Reaper at all, but during the render.....it blasted away at +10db and the cpu useage was off the chart. It didn't do that the first time with the render ahead size at default 200 - it didn't go into the red during render, but still had the pops and clicks.
I have no idea why the above happened, but.......before doing this reaper render I imported all
the tracks into the Mel Bridge, not just the vox tracks, worked out how to use GarageBand Au effects in Mel Bridge, saved audio as stereo mix wav file and.....perfect. I converted the wav to mp3 using the Switch app and still perfect - well, the recording anyway, the vox tracks, not so sure.
From now on I'll just stick to using the cre8 directly, but I'm letting you know what happened in case it's useful. I'd also be quite interested to know what is going on in Reaper using Mel Bridge - I wasted about two weeks on this.