Unless you are completely committed to using those older Shimano STIs, here is something "completely different" to consider. Gevenalle shifters are available in both 10-speed road and MTB format for 1X, 2X or 3X cranks. For that matter they are also available in 11-speed and you can use an 11-speed MTB cassette on a 8/9/10-speed freehub. These have external cables but so do your STIs.