How to output high quality 360 VR video with Premiere Pro?
Format: h.264 -> that's what makes it look bad.

h.264 is a codec in the mp4 container and always compresses, making the file smaller and look worse. For lossless exporting you use prores 4444 xq if you're using mac (or windows with the newest version of premiere) or DNxHR. I highly recommend prores 4444 xq though.