I think my problem is the xrandr --output command runs either too soon or too late? This has been mentioned in various sites like here and here.
I took advice from this comment in a thread about changing DPI with xrandr and added the line bash -c "sleep 15; xrandr --output LVDS-1 --scale 1.6x1.5" to a custom Startup Application and this did work so the resolution was set to 3072x1620 on startup.
In fact changing the resolution this way was then interfering with a Windows VM guest I had running in Virtualbox so now my current thinking is to stick with the 96dpi 1920x1080 mate gives me by default. Just seems hard on my eyes... especially the menus and URL bar in Brave Browser that seem too small and I don't know how else to make them bigger.
Flex