My friends had this issue when they decided to spend six months cycling around Europe. They ended up having to re-schedule a lot of their trip to stay outside the Schengen area. At the time, they went to Croatia for almost two months and really enjoyed their time there, but Croatia was in the process of being admitted to Schengen. That was back in 2015, so I would double-check your information about Croatia not being in Schengen yet. I thought they now were.
As for your first question, if Bulgaria is NOT part of Schengen, yes, you could do 30 days, leave and then do 60. What you can't do (and a lot of people try) is re-start your time in Schengen every time you leave. For example, one of the first idea my friends considered was popping over to Morocco for a short visit. However, that does not re-start your 90 day limit.
You can also apply to get a visa to stay longer in one country. My friends considered doing that for France (which they originally planned to spend longer in). It was possible, but it required a lot of work (they needed things like signed bank statements, etc) after leaving Canada was pretty much impossible. So perhaps look into getting a longer visa for the country within Schengen, where you plan to spend more of your time.
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