Have you tried taking the time instead of asking for it? It works surprisingly well in my experience, with very little pushback (and when there's pushback, it's important to understand why and address it).
The first few months you focus on delivering value without rocking the boat. You probably don't have enough context to make any meaningful changes anyway. Then when it's time to groom stories, you select the time it takes to do a proper job, to make I improvements, and as long as it's not ridiculous, who's going to challenge you on it, and based on what?
Of course, you need to pick the right battles. Sometimes that hack to fix a production issue will stay like that for ages, and the proper fix will never be done, and that's the way it is.