You don't have to delete your post once it got answered, users in the web who are looking for support may be able to use it for reference.
1st question the description for RBF transactions is pretty vague so I'm unsure if I should enable that setting or leave it disabled?
It's to flag your transactions as "
replaceable" but only until it's mined, useful in cases where you need to increase the fee by replacing it with a higher fee transaction or if you've sent it to the wrong address.
If it's enabled, "
increase fee" and "
cancel transaction" right-click options in the history tab will be available for txns created after it's enabled.
The downsides that I can think of are: some services that doesn't support RBF may not credit your deposits if you've used RBF to increase the fee;
and some exchange/casino with zero-confirmation deposits wont grant that feature to transactions with RBF flag.
2nd question is should I disable "output value rounding" as it says at most 100 satoshis might be lost due to this per transaction but I don't know how much USD loss that would potentially be?
One satoshi is equal to 0.00000001 BTC; so "
at most" 100sat is currently about 4 cents (
USD).