koala logs

I like the terminology you're using, less confusion!

I started out with concave back like OP due to factors like excessive reach due to long stem for instance. Eventually I nailed down my bike fit. Reduced reach with shorter stem made the convex lower back posture easier. Note it doesn't compromise aero as long as you didn't change the stack height. Great results for me with reduced reach and adopted convex back posture. It fully unloaded my arms when maintaining tempo effort and also reduced strain on my lower back. It definitely improved comfort. I also improved power output at the same time but can be to other factors like adopting better pedaling technique and better training plan. I also actively recruit my glutes. I agree it's an effective means to avoid knee injury because it helps spread the load between the knees and the hips. Although it may increase load on the hips and the core muscles as well.

I've also ridden with slightly convex back at some point when I'm training to transition between the concave back posture into convex back. The effect is literally in between. Better than concave back, less strain in the core muscles and arms but still more strain than fully convex back. However, there are many riders who are perfectly fine with slightly convex back posture. It wasn't perfect for me but wasn't bad either. If flexibility keeps you from getting into fully convex back posture, then it's a good alternative. If you're having issues with it like increased lower back strain than you may need to try a more upright riding posture or simply do core workouts to improve core strength. It's possible your active recruitment of the glutes is increasing strain on the hips and that will inevitably strain the core muscles as well. In that case, the solution is exercises to improve core strength.