Addresses #29283 (review).
Fixes a JSONRPCException: Invalid amount (-3) exception by ensuring the amount sent to sendtoaddress is rounded to 8 decimals.
See https://cirrus-ci.com/task/5562947183837184?logs=ci#L2559
Note: since round can also round down, min_amount is not exactly guaranteed, but this is not a problem for the current usage. I've added a docstring to highlight this.