What Bitcoin node do they pull? And how often would they - remids me of traffic reduction on certificate/revocation servers - same problem. Also there are many different slow/medium/fast fee estimations. Or how long does the mempool have to clear up for notifications to trigger.
It can be done for sure. But doing it such that it it's a good UX takes effort
The problem with that is that you can't delete a tx from the mempool, so if you ever need to move funds elsewhere you'll have to do a CPFP which is even costlier.
If you are patient you can be rewarded. Your transaction can also wait for months. I've had this happen but it wasn't a big deal. I was experimenting anyway. Just a word of warning.
One note, I have really not found a good way to do this other than just keep an eye on them manually. The services linked below tend to have smaller mempools and lack accuracy to the detail I'd like. Mempool.space is the most accurate place I have found to track fees and that's what I tend to use. I found the ntfy service to be way off on the low side. If you watch fees though you start to see patterns when they drop and rise based on time of day and time of week but there's no hard and fast rules.