Decal count has never made any difference to me to being able to upload a livery - and judging by some of the liveries out there, decal count is unlikely to actually be something they're not handling properly.
I was experiencing repeated problems not long after the games the launch, no liveries would upload at all - didn't matter if it was a single decal livery or something with 100's.
One day, I plugged my PS4 into my router with a network cable and I've had 100% success since literally that moment.
It's not that my internet was bad, it is well above average for the UK, however the game seems very sensitive to the speed the PS4 is connected at. My typical upspeed is about 17mbps, for some reason - over wi-fi, the PS4 was well down in the kpbs range - using a wired connection fixed that - and that fixed my upload issues immediately.