As I was scrolling down I was halfway toying with the idea of suggesting stem shifters before I had even seen that post but I had already ixnayed it, stem shifters are also inconvenient and can be in the way, if for some reason you found thumbies unacceptable I'd be inclined towards bar ends actually even though I know that goes against your current miyata grip setup. Thumbies should be just fine I think, but my personal bias is I like having my brake levers and shifters immediately at the main hand position for a bike.