That's far from typical hardware to deploy maria/mysql on, and far from realistic workload.
1M transactions per second with 100 % reads might as well mean 10K transactions per second with 95 % readsm 5 % writes ("the usual OLTP workload" per common wisdom) and 1K transactions per second on 50 % reads 50 % writes. Or even less. Or much more. It's difficult to even quess.
That makes me wonder if a smaller number of tables would have hit contention even in a read only workload.
Well it's not the only database that has this problem. I wonder if partitioning is enough or if you really need separate tables.