That's not necessarily a mistake- you can do a wash over an unsealed paint job and Tamiya paints are pretty durable once dry. But..... unless you're using a high-gloss paint to start with, you'd want to do a gloss coat of some kind before decaling to avoid silvering and other nasties that happen to decals. Also, flat paints will trap more of the pigment particles and take on a color change and a dirtier aspect than will gloss paints.
You should do washes and weathering after the decals so that they blend into the finish instead of looking freshly painted-on over the weathering (I learned this the hard way...).
There's so many things that we learn on this model so that the next will be, hopefully, better.