Going flat

These last few weeks I have been working on getting my paint surface 'just so'. I want even color with no visible brushstrokes, sharply defined edges between colors (especially when those colors are very similar) and a matte finish.
The matte surface can be taken care of at the varnishing stage but the rest (I suspect) is down to a mix of the paint, the brushes used, the painting surface and the brushwork itself.
So far I have been investigating the paint mix, testing every medium I could think of with fluid acrylic. All the paints and mediums I used are Golden acrylics.
Self-leveling medium was (unsurprisingly) the best at this but was also very hard to brush. Plus it is very glossy and would probably need several matte varnish coats to get an even matte-ness.
I tried a self-leveling/matte medium mix but rapidly lost the leveling nature.
The second best was GAC500, which Golden list as being for self leveling applications. It also dilutes fluid acrylics without changing their properties so it was very brush-able. I still got some brushwork ridges, but it was less sticky and viscous than self-leveling - much better to work with.

Of course, I could mask and spray, or roller, to get a brushstroke-free surface but I enjoy the moving meditation involved in this even, smooth brush painting. A painting equivalent of the hiker's "leave no trace" perhaps.

I used a transparent color (dioxazine purple) for all the trials and one thing became very clear: it is close to impossible to get a completely even coverage with a transparent color, whether I tape the edges or paint them freehand.