The -maxtxfee (which is put into m_default_max_tx_fee) is documented as "the maximum total fees to use in a single wallet transaction"

This seems true in many instances when we look at the code:

But there are also places where this value is used as a feerate per KvB:

I don't think these instances of conversions to feerate result in anything very problematic, but it doesn't make sense to just convert this fee to a feerate because not every tx is the same size. I can see why a wallet user might want a maximum fee and maximum feerate. So my suggested solution is to have both: add a -maxfeerate and put those checks under the new value.