Hi.
I don't know of any mobile device that is still around the 320px range.
320px is still a minimum width of which should be focused on. Since starting with 576px to less the layout becomes rubber and you must have a reasonable supply to work with this. Using this width, you will be sure that your site is correctly displayed on all devices, even if their screen is a little wider.
By the way, iPone 5s = 320px. In the world, there are still many users have this device.
In general, it took quite a lot of time (several years) and the requests here on the forum, so that the Joomshaper team finally makes an adequate representation for testing on mobile. And now it is completely adequate and corresponds to the Bootstrap (5) breakpoints. No need to change anything in this. You just need a little more knowledge of CSS layout and Bootstrap breakpoints to understand this correctly.
The breakpoints should be universal, and are not tied to the width of specific devices, which allows you to make sites that work well on all devices with a minimum amount of code.