I brush painted with enamels for years until I could afford an airbrush. Usual advice Bob, dilute it until it runs nicely off the brush…..I would dilute it in a separate container though. Mix the enamels well, take some out and dilute it for brush painting. if it’s too thin, add more paint. If it’s too thick, add more thinner.
Once you have a mix you like close the enamel tin to stop thinners evaporation from the bulk. I’ve always treated my enamels like this and have some that are still as good as new after about fifty years. One thing though, enamels need to dry overnight between coats. They are very easy to “reactivate “ with more thinners even when dry.
Rinse your brushes with white spirit, it’s much cheaper than own brand thinners. After they have the colour removed, wash the brushes with dish soap to get rid of the white spirit. Rinse under running water.