That's what Python is for. Or Ruby. Or Perl.
Python I know something of. I find it rather painful to use. I suspect, though, since LibreOffice uses python scripting, I could figure out something, and hopefully not come up against the TOS and Admin/Mods of Sufficient Velocity while doing so.
And therein, as they say, lies the rub...
A bit of a description of the workflow:
1) I write in Libre Office.
2) I run things through the native spell check
3) Then Grammerly.
4) I redo the formatting, because Grammerly destroys it.
5) Using a self written BASIC macro, I convert it to BBCode for one of the two boards I post to.
6) I post to one forum using copy and paste.
7) I check a checkbox on the UI for the macro.
8) I post to the other forum using copy and paste.
9) As typos/corrections come in, I apply them to the master document, then loop to 5)
10) If there are none, I relax, continue writing, and subtly troll my readers; I also post witty comments in other threads I'm reading. I also engage in attempting to overheat my PC doing 3D Rendering of realistic fur and hair, get tortured by my physcial therapist, do my chores,
etc.