Generally you want to do all your painting and decaling first before weathering
The reason is you want those to be a part of the weathering
For example, if you have a dirty grimy area, and a pristine clean decal is placed over it, the decal will look odd unless the intent was to show someone cleaned off the decal area. Maybe that is appropriate for something to see a registration number or something
likewise, a painted area should be done first unless you want that painted area to look like it got a fresh coat of paint
One thing to make sure with the decals is to gloss coat the area you are applying it to, then give then a day or two to dry fully, then seal the area again with some gloss coat to protect it from any weathering steps
An alternative I sometimes use when I am concerned the weathering process may damage the decais is to do all my really heavy oil weathering first, since that process can involve using turpentine washes and lots of rubbing with a towel/cloth, then when dry, seal it with gloss, apply decals, and then do a final light pass of weathering/mist coating