Sorry you have a misconception about eFuses there. Basically, after most, not all, updates, your Switch will burn a number of eFuses, that's supposed to happen. So then every firmware has a specific number of blown fuses associated to it. You're on 5.0.1 right now, so I think you have 5 blown fuses now. Say you would now downgrade to 4.X, which usually has 4 blown fuses (not sure if these are the exact numbers but I just want to tell you how it principally works) , the Switch would then recognise that there is a too large number of blown fuses and would therefore refuse to boot.
Regarding your question about updating to 5.0.2, you might as well do so. As long as you're above 4.1.0 you won't be able to use deja vu, the unreleased exploit anyway. You could still do that update after running SX OS though, doesn't make a difference.
Game cartridges always ask you to update, they won't install anything automatically.