¡Solved!
It is totally crazy, but it worked.
By improving that trick, according to that instruction on the Manual 3.2 Blender, that you need to have both Vertex Groups (.L/.R) already created, and that the Mirrored side requires to be completely empty,
With correct symmetrical names (.L/.R) on Bones and Vertex Groups, I’ve deleted one my antique special Vertex Groups, and instead, I’ve used a Bone to generate Weight Paint to a single Vertex (with Weight = 0, Blue) on the Mesh Object; then I’ve Symmetrized that Bone (a symmetric Vertex Group was generated, probably with Weight 0 on that Mirrored single Vertex, because just before, that was just and empty Vertex Group), and by Applied the Mirror Modifier, the setup was mostly complete. I’ve had to adjust all the Bone Constraints according to Vertex Groups of both sides (.L/.R), but this step can be done before Applying the Mirror Modifier also; actually, as soon as you have Symmetric Bones and the symmetric Vertex Groups (even if the Mirrored one is still empty). Changing this last order of things isn’t a problem. This automation only works because those are “Weight Paint” Vertex Groups and have same names as their corresponding Bones; all in Symmetry (.L/.R).
Note: My case is so weird because the same Vertex Group I’m using (that normally would be used to be Deformed by the Bone) is actually being used to drag its Bone around (through a Copy Location Bone Constraint; Targetting its own Weight Paint Vertex Group). If this Weight wasn’t 0, I believe it would break the system as this is what already happened before. Now, I must just be assure that this 0 Weight will not interfere in any grave way with Weight Painting on the rest of the surround Mesh; but, apparently, this stuff is just working as expected. If I find any unexpected behavior I guess I’ll be posting here.
Cya.