Basically, Meikon is targeting a few specific use cases:
- Sony 16-50mm with or without wet lenses, optionally behind dome
- Sony 10-18mm with flat or dome port
- Sigma 16mm f/1.4 with dome
- Sony 30mm macro
- Sony 90mm macro
Note that manual focus is possible only with the Sony 16-50mm, everything else is AF-only. Nauticam targets a great deal more use cases, but at an order of magnitude higher price - and even they don't bother targeting manual lenses, besides the old water-contact Nikkors. A Meikon small dome for fisheye lenses is supposed to come out next week, and it'll likely work with your 7.5mm Rokinon, but as I mentioned, it's very unlikely to offer any lens controls at all. It may be possible to DIY a lens gear to control either focus or aperture, but definitely not both.
Also, Meikon reps have mentioned that they're working on an A7III housing, currently in prototype stage - it may come out in September, though if past promises are any indication, this is likely to slip by 3-6 months.
Edit: As far as costs for a new system go, assuming you're willing to buy used - An A6300 body is about $700, 10-18mm is ~$500-600, 90mm macro is ~$700-800, Meikon housing with ports is currently $550, tray with a pair of arms and four clamps is $140, pair of ST-100 strobes is $510 - total about $3200; add a few hundred more for memory cards, batteries, bags, protective covers, spare o-rings, etc.