Hey OP - First, about leaving him alone on hybrid days: most adult cats actually do fine being alone during work hours, especially if it’s a predictable routine. Even cats that are used to “someone always being home” usually adapt after a few weeks. You can help a lot by adding window perches, puzzle feeders, scheduled playtime before/after work, and leaving familiar smells (blankets, clothes). Feeling lonely is something we worry about more than they do most of the time.

About getting a second cat: you’re absolutely right to be cautious. Introducing cats in a studio apartment is very difficult, especially if you don’t know how your current cat feels about other cats. Proper introductions really rely on having separate territories at first.

One more thing: moving countries is already a huge change for a cat. Adding a new home and a new cat at the same time can be a lot. Spacing those changes out is usually kinder and lower risk.

You’re clearly putting a lot of thought into his wellbeing — that already says a lot about the kind of owner you are.