I said it many times, you can guard an ice cream shop with a guy, you can guard a Walmart with two armed bodyguards, but I doubt guarding the White House with 3 guys is a good idea.
If you want to be protected by more Proof of Work, then say it explicitly inside your script:
OP_SIZE <difficulty> OP_LESSTHAN OP_VERIFY
OP_CODESEPARATOR
<pubkey> OP_CHECKSIG
Or even, you can make it signable in a very similar way, as existing P2WPKH outputs are:
OP_SWAP OP_SIZE <difficulty> OP_LESSTHAN OP_VERIFY OP_SWAP
OP_CODESEPARATOR
OP_DUP OP_HASH160 <pubkeyHash> OP_EQUALVERIFY OP_CHECKSIG
And then, it is up to you, how much Proof of Work you want to require in your transactions. Then, even if the main chain is protected by regtest difficulty, double-spending your transaction can be very difficult, and require a lot of Proof of Work.
For example: try double-spending this transaction:
https://mempool.space/tx/8349df0753e80cce322322f1b76789e1d0fd6693aed2f4de4e49576423081ae7See? It is difficult. First, because it is covered by mainchain Proof of Work, and second, because even if it wouldn't be, then you would still need a few days of grinding with GPUs, to even produce an alternative valid version of that transaction, and even move the same coins in regtest.
So, it is up to users, how much Proof of Work they will want to spend in the future, to protect their own coins.