Bill, the main one is Lighthouse usually simulates 3G speeds. Mobile browsers are more picky and use more CSS browser code to render, usually around image resizing, and phones are not are powerful as desktops.

I recently engaged with Xara about it XC provision of HTML email templates.
email clients are very fussy and for Xara to achieve this is a great boost for those whose clients want email as one of their provisions.
I notice one thing that I asked Xara about - the Xara design uses the table tool but the rendered output is an HTML <table> tag construct, a thing i have been chasing for years.
No answer from Xara if it intended to change the Plus applications HTML Filter plug-in to do the same.

The other thing that blew me over was the HTML rendered without any of the standard JS and CSS support files. It was just html plus images.
Consequently, Lighthouse gave 100% Desktop and 92% Mobile performance. The images remained a PNG/JPG regardless of settings to WebP.

So sometime, a lightweight mobile render may be on the cards.
Perhaps too main and Variant will disappear and were have Main in the direct URL and the 'variant' as a separate render in a sub-folder (.../m/...).
Code would be needed to toggle between or just a button Desktop/Mobile Mode.
Much cleaner.

Acorn