I carry a back up mask on cave dives, but I wouldn’t bother on OW no-deco dives, solo or otherwise. I can easily and safely ascend to the surface without a mask. I have had my mask kicked off by an enthusiastic thrasher on a crowded reef, but I just put it back on. Worst case, if you needed to see your gauges you could hold the mask in place if the strap broke.
I’m not saying its a bad idea to carry a back up mask on all dives, but for the type of OW diving I do, it’s just not something I personally would need. Someone in very cold water, on a deeper semi-technical dive, sure I can see it. And for cave diving or other overhead, it’s a requirement.
What’s probably more important for an OW recreational diver is to practice getting comfortable without a mask on for a few minutes, or at least with a flooded mask. That’s a skill you’ll need regardless of whether you have a spare or not.
I had a friend who used to carry a pair of swim goggles as a back up mask. He would clear it using his regulator and could only ascend, but it did sort-of work for him.