The way I approached titration leaned on my own outcomes plus anecdotal reports from this community and elsewhere.
Given there are people who barely respond, some who respond intensely, and everyone spread between, the eventual maintenance amount could realistically land wherever your final increase happened to be.
Makes sense to stick with the smallest amount that keeps steady progress going rather than rushing higher.
On Tirz I've seen folks hit their target under 5mg while others need upwards of 30mg+
Whether you've hit a wall really depends on the individual, but personally I've held at 15mg for seventeen months and my weight's barely shifted a few pounds in either direction.
Could call that a wall, sure.
Though I've also loosened my calorie intake and started eating more freely.