Change difficulty using bitcoin-cli/bitcoind in regtest mode
No there isn't. Regtest is not designed to test mining ASICs and the motivation for such a RPC seems limited given regtest is for local testing.
However, if you want to experiment with mining on a network with limited network hash rate you could either try mining on the default Signet administered by Kalle Alm and AJ Towns or you could set up your own custom Signet network and then use the Signet mining utility in this PR (19937) to mine blocks. AJ Towns lays out some examples in a PR comment.
e.g.

Generate blocks indefinitely, at 10 minute intervals, paying block
reward to the given address, using bitcoin-util to do multi-threaded
proof-of-work:
 ./contrib/signet/generate.py --cli='./bitcoin-cli' generate 10 --block-time=600 --address=&quot;tb1...&quot; --grind-cmd='./bitcoin-util grind'