Thanks Tony.
For the record, I've decided to take the following route:
- Open the shadow tiddler $:/core/ui/Buttons/more-tiddler-actions
- Modify the list filter inside of this shadow tiddler so that it excludes the list items I don't want to see
I've also made a copy of the modified shadow tiddler's text field in a separate tiddler in case I ever need to bring back my changes.
If I ever need to bring back the "factory" version, all I need to do is delete the modified (overwritten) shadow tiddler. That's the beauty of shadow tiddlers in general.
Now I only have the stuff that I need:

Thanks everyone for your help. Issue solved.
By the way, the built in class .tc-drop-down causes the More tiddler actions dropdown to overflow viewport on mobile, because its min-width is set to 380px for some reason. Most mobile browsers in mobile view have a viewport width of 360px today, so I've targeted that with a media query in my own stylesheet like below, but that's an unrelated note to anyone interested:
@media (max-width: 650px) {
.tc-drop-down {
min-width: 360px;
}
}
Regards,
Hubert