Depending on your skill level, drilling counter sinks to use an outer ring for a middle ring should be very doable. There are a lot of countersink drill bits available and aluminum is easy to drill. My main concern would be not drill too deep but careful drilling should do the trick. Use an old ring or even buy some aluminum stock from a hardware store to practice. Worse case is you ruin a ring and have to by another. Before drilling however I would try the ring as is to see if it works. With a 10 or 11 speed chain it may work just fine.

I have used a lot of customized cassettes before finding our current Shimano 12-28. The all Shimano cassettes do shift a little better but many custom cassettes work well. I like the OP's approach to gearing. Find what works for your team and try not to be limited by what Shimano thinks is best to manufacture and sell.

Is this a 11 speed project? If so I would also try a Camy cassette. They are expensive but have some interesting cog combinations. 11 speed spacing is supposed to be very very close for Campy and Shimano. The OP's top end gearing is similar to our top end and we can use a cassette a long long time.

Is there a possible issue of damaging the belt if the chain is dropped? Even with an all chain system, having same side drive with two chains crossing always appears to me be a tangled mess waiting to happen.