I made two Bitcoin transactions in Trezor Suite with RBF enabled.
1’st transaction:
BTC sent from two Inputs to one Output with option “Send max” enabled where Inputs are from one wallet, and the Output is from another wallet (another seed-phrase). After some time, I wanted to increase the fee to speed up the transaction, but there were no free funds at the input addresses, but Suite offered to increase the fee nevertheless, and I used it. As a result, the fee was taken from another last address which received btc of the same wallet.
In this case, could Trezor offer me to choose an address from which funds would be taken to increase the fee? The Trezor Suite just offers to do this, but does not show which address the funds for the fee will be taken from, and even more so does not allow you to choose any other address for this.
2’nd transaction:
BTC sent from four Inputs to one Output with option “Send max” enabled where Inputs and Output is a same wallet (same seed-phrase). After some time, I wanted to increase the fee to speed up the transaction, but in this case Trezor Suite don’t allow me to increase the fee, the Replace transaction button was disabled (grayed). I have enough btc on different addresses of this wallet but I can’t bump fee with using btc from another address as in 1’st transaction. Perhaps, to get out of this situation, I should transfer a little bit of btc to one of the Input addresses of the incomplete transaction to enable state of RBF. I tried to send some quantity of btc to one of Inputs (second address) and still can’t bump the fee 
So, my suggestions for improvement:
Allow to select an existing address of the same wallet for using btc to increase the fee if the “Send max” option was used or if there are not enough funds for the required fee at the address. Display more information about which address will be used to increase the fee. As I see it, RBF works differently if the transaction is external and internal.
I’m sorry if I explained everything in a muddle.