Some months ago I proposed an alias, but the amount of tags for choking_on_cock is just 37.

I think the alias would still be useful for the future, but would it be OK for me to replace manually in the posts right now? Looks like this change in particular is not controversial...

If this is OK, in which moments would it not be OK to make these kind of changes, and be better to wait for an alias approval? Could I do flexing_bicep -> flexing_biceps (332 posts) (thread) if I had the time?

Normally, I would not suggest blanket tagging (i.e., mass adding or removing tags) due to the propensity for error.

However, considering that these are just basic aliases, you can try to manually change all the tags by yourself.
Bear in mind though that it's entirely impractical (since it will eventually be aliased) and may take a while for a normal member with a 150-posts hourly tag edit limit.
Priv+ members don't have these restrictions and can utilise tag scripts while only moderators can do true blanket tagging.

Alternatively, you may change the Mode menu underneath the search bar from "View" to "Edit" to help make tag edits easier (by tagging though the thumbnails), but you will need to be careful of mistags.

What I wouldn't suggest though is making mass tag edits by yourself and without community discussion/approval.
For example, your alias/implication suggestion is currently being disputed/argued upon but you take it into your own hands to change the tags anyways.
This would be considered Tagging Abuse / Tagging Vandalism as per the Code of Conduct.