1. A lot of problems arise from too few people working on too many things. If it's one-two devs and backlog is growing, the problem is not that you have no time to fix things, but that you're understaffed. If you have enough people, then from the business perspective it shouldn't even be that noticeable that someone is refining previous work, while someone else is building the next thing.
2. If you're not understaffed, then the best time to clean up new code is during or immediately after writing it. A phrase I like to use is "while it's still fresh in memory". You're saving time and not adding new bugs, by not having to remember everything again, load all that context back into your head.