I have that exact setup and I don’t experience reboots.
try my kernel parameters
options quiet splash iommu=pt
options loglevel=3 radeon.si_support=0 amdgpu.si_support=1 radeon.cik_support=0 amdgpu.cik_support=1
options amdgpu.vm_fragment_size=9 amdgpu.audio=0 amdgpu.aspm=0 amdgpu.ppfeaturemask=0xffffffff
