I had the same problem and ended up building a Cloudflare Workers solution that handles subdirectory mounting + hreflang cross-linking between two Ghost instances. Sharing it here in case it helps: Bilingual Ghost with Cloudflare Workers
I would like to share our solution for the non tech-savvy people here:
TanaFlows theme which works multilingual, also uses Synaps Media. You can check its demo: https://tanaflows.bepublish.com And @cuongtran offers 35% discount for Synaps Media customers.