On the odd occasion I run Resolve on my laptop, I sometimes see the image stuttering like that. Basically it is because my GPU is below spec. Your iMac probably also has a GPU below recommended spec for Resolve. The problem trying to run 4k footage for example boils down to the VRAM on the GPU.
Yes Resolve can run Sony codecs directly without transcoding so the only reason to transcode is if you need to run smaller proxys on an under powered computer like 1080 instead of 4K or bigger frame sizes