My first suggestion would be to replace the triac with a solid state relay. The triac is a bipolar device, and has a significant voltage drop. This means that power is dissipated in the device causing it to heat up. Solid state relays have very low on resistance so they dissipate very little power. You can also get them with high enough current ratings that the transformer for the AC could not provide enough current to fry them. Also in most cases, the input is an LED, so with a current limiting resistor for the LED, there is no chance of frying the drive circuit. An example of a part like this is this: https://www.digikey.com/product-detail/en/toshiba-semiconductor-and-storage/TLP241AF-TP4-F/TLP241AF-TP4FTR-ND/7652708
Yes, this would increase your BOM cost by about $10, but that is better than burned boards.
These parts are also completely isolated, so one side of the AC would not need to be hard wired to GND, and you could use a full wave rectifier instead of the half wave rectifier you are currently using for the 5V power supply. It would be nice to have a big enough power supply to power a Raspberry Pi 4 with a few peripherals.
Resettable fuses are so cheap you could use one one every channel: https://www.digikey.com/products/en/circuit-protection/ptc-resettable-fuses/150?FV=69%7C409393%2C686%7C150723%2C686%7C150724%2C686%7C153583%2C686%7C157293%2C686%7C160817%2C686%7C213650%2C686%7C257642%2C686%7C257644%2C686%7C257645%2C686%7C287637%2C686%7C287639%2C-8%7C150%2Cmu1.1A%7C683%2Cmu1.2A%7C683%2Cmu1.5A%7C683%2Cmu1A%7C683%2Cmu2.2A%7C683&quantity=0&ColumnSort=1000011&page=1&stock=1&rohs=1&nstock=1&k=fuse&pageSize=25&pkeyword=fuse