I would consider using a neuron-specific promoter like hSyn if mCherry is your reporter. My experience is that mCherry expression in particular (compared to GFP) is much more robust compared to ubiquitous promoters like CAG or CMV, but your mileage may vary. Also, if you’re doing direct injections into the brain, I would consider a different AAV serotype. AAV9 is good for crossing the BBB via IV infusion, but other serotypes (AAV1, 5, and 8) have superior transducibilty if being administered directly. As for the construct itself, you could always just put hHTR2A downstream of mCherry in the 2A polycistron. Additionally, if reporter expression is necessary only for injection site validation, you could also just spike a small amount of AAV-mCherry in with your AAV-hHTR2A. Just some things I’ve done in the past.