I just use the same configuration I would use with other divers; it depends on the environment. Relatively shallow open water? Single tank, no pony. Deeper-but-still-recreational OW, I might sling a bailout bottle but I very rarely do those kinds of dives solo; I just don’t find myself in that scenario. For solo cave dives, which I do on occasion, just my regular SM set up. I still go through the pre dive checks in the same order, write things down, and stick to familiar caves.
I really believe that people focus on gear too much in these conversations. Dive safety, regardless of solo or team oriented, is always about diver behavior, judgment, and awareness. Of course being prepared for gear failure is a part of the picture, but there are many other things to manage.