If you want to stay with the single DIN, try one of
these Alpine units. Alpine designed them specially for iPods.
I had one in the Lotus Elan I used to drive, and it was great.
I've been using a double DIN in the Boxster ever since I got it. That was pretty much the first mod I did, and I've stuck with Pioneer, though I've had to live without there being volume know -- just buttons. Since the first double DIN unit I put in it was a Pioneer, and then I moved that one to my truck when I wanted to upgrade, so both the Boxster and the Truck have the Pioneer harnesses in them, so I've just stuck with Pioneer. Right now I have an 1400NEX unit I really like, and I've just installed a wireless charger and a CarPlay2 Air wireless CarPlay adapter.
As for having more than one USB, there's no reason for the stereo, only for additional charging if you have another device that needs it. I replaced the cigar lighter with a built-in Dual USB charging port, and in the round blank opposite that, I put in a USB and Aux jack which I connected to the rear USB and Aux outputs on the head unit. You can find both of those on Amazon pretty cheap.
For one with a volume knob, which seems important to you, and I think is definitely convenient, I would maybe go with Sony. Most of theirs have a knob. Like
this one.