Have you tried a CRF of 19 or 18 for HEVC? I see your CRF is 15 and from what I understand for HEVC under a CRF of 20 you can’t really see any difference. If you haven’t already, try them and see how the video quality turns out so that way you may get even smaller file size with still the same video quality.
I also see that you used the same CRF for both HEVC and AVC. The CRFs for HEVC and AVC are not 1 to 1. The equivalent CRFs for HEVCs are lower than AVCs simply because HEVC is about lower file sizes. That’s why the CRF for the default HEVC preset in Shotcut is 28 while for the AVC presets they are 23 because that is the equivalent of HEVC’s 28. You would get the same video quality at 28 in HEVC only in a smaller file size than AVC’s 23.