Every time I switch brush type in Affinity Designer, the stroke size changes as well. How do I prevent this behaviour, keeping the same brush size when changing the brush type?.
Double-click a brush in the Brushes palette. You get an editing dialog for brush properties. One of them is the default width.
Making a brushset with proper defaul widths is your only option, there's not available such option as "use previous brush width" or "set the brush width for the painting tool". Unfortunate, but true.