If anyone ends up here. I have created a yeoman generator that updated the regular spfx webpart project to run with webpack 5 and latest version of Aurelia2.
Install generator with:
npm install -g generator-aurelia-spfx
Create you webpart project with:
yo @microsoft/sharepoint
Then run:
yo aurelia-spfx
And answer yes to all questions
Start one terminal and run
gulp serve
Start second terminal and run
gulp buildwp5 --watch
Add webpart to the workbench and start playing with Aurelia2 and Spfx webparts