Hey All!
I know it's been some time since this post, but I just ran into the same issue with my fiancee's new iphone 12 mini not sending pictures to my Motorolla android phone. I fixed it =)
We have Cricket Wireless as our carrier but I'm curious as to whether or not this fix will work on other carriers.
After trying the usual enabling MMS, disable iMessage, reset network settings recommended by every google search result I looked at, I went into (on her iphone 12 mini) Settings> Cellular> Cellular Data Options. Left roaming "Off" but I switched the "Voice and Data" option from LTE to 4G. And instantly the problem was solved.
I believe this is because her iphone supports 5G (assuming due to the fact that LTE would be 5g now, I assume) while my Android does not. Or, the LTE signal in our area is not strong enough to send the message without timing out, not entirely sure.