This may sound strange, but... the same thing kind of happened to me last night when my HomePod's were updating to 18.3. While all the others in the house updated just fine, one of the mini's got stuck in a reset loop with the red spinning light and Siri telling me “Your HomePod is about to reset, keep pressing till you hear 3 beeps.” The 3 beeps would immediately play and the mini would shut down, turn on again, and have a white swirling light that turned red, and say the same thing. I unplugged it and waited 10 seconds, then tried it for 30 minutes, then several hours... same thing. Even after doing the update and restore from plugging it into my computer, the mini displayed the same problem.
After doing a search online I found the following solution. And, unbelievably, it actually worked! The HomePod's get dust and (in my case, greasy dust, as it is in the kitchen) on the glass tops. I noticed mine needed cleaning and I had wiped it off, but that didn't really do anything, nor did holding my finger down for several "reboots." But, strangely, after taking a small dusting cloth and spraying it with Windex and rubbing the top in a circular fashion after it rebooted the light stayed the swirling white. It went red a couple of times but came back to the swirling white. I kept rubbing the pad over the glass top for about a minute, maybe even two, and then all of a sudden I heard the set-up sound, and Wholla! I was then able to go through the set up process on my phone without difficulty.
I think, in regards to the volume issue, what you may be experiencing is the often reported "ghost touch" issue. This may be, as with mine, related to dust particles that have settled onto the device. Apparently, "cleaning" it the way I described above resolves the issue. Or, at least it did for me... and many others on the reddit thread I found this fix on. It sounds crazy, I know. But, you might want to give it a try. Best of luck!
