For me, personally, I go geared or fixed. Singlespeed with a freewheel… I can just not shift my geared bike.
Overall, I have found that I don’t lose much speed on my fixed gear. It’s a bit more taxing than geared though because of the cadence swings but it’s also fun. Finding the right ratio for your legs and terrain can take some experimentation though.
I got into fixed by just buying a converted steel road bike off Craigslist. If you aren’t up to building your own, I’d just pick a mid range gear on your bike and then wrap some electrical tape around the shifter to remind you not to shift. A singlespeed will be a bit quieter with the perfect chainline and no derailleur pulleys, but overall I think if you hate riding a geared bike in one gear, you would hate single speed also.