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”
https://github.com/bitcoin/bitcoin/blob/632a2bb731804dffe52bd4cbd90bfee352d25ede/src/wallet/init.cpp#L63
This seems true in many instances when we look at the code: https://github.com/bitcoin/bitcoin/blob/632a2bb731804dffe52bd4cbd90bfee352d25ede/src/wallet/spend.cpp#L1293-L1295 https://github.com/bitcoin/bitcoin/blob/632a2bb731804dffe52bd4cbd90bfee352d25ede/src/wallet/feebumper.cpp#L114-L119 https://github.com/bitcoin/bitcoin/blob/632a2bb731804dffe52bd4cbd90bfee352d25ede/src/node/transaction.cpp#L71-L79
But there are also places where this value is used as a feerate per KvB: https://github.com/bitcoin/bitcoin/blob/632a2bb731804dffe52bd4cbd90bfee352d25ede/src/wallet/wallet.cpp#L3061-L3064 https://github.com/bitcoin/bitcoin/blob/632a2bb731804dffe52bd4cbd90bfee352d25ede/src/wallet/rpc/spend.cpp#L433
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.