I was under the impression that 思った meant that, sometime in the past, you did the instantaneous action of "think" and had a thought while 思っていた meant that you kept a thought for a period of time. However, I did a search of "ずっと思った" and found that there seemed to be an overlap with "ずっと思っていた" with sentences such as "マンションに住んでよかったってずっと思った."
Your interpretation about 思った and 思っていた is CORRECT.
You usually don't say ずっと思った。, especially the use of ずっと is incorrect. The only possible interpretation of
（何かをする度に）「マンションに住んでよかった」って 毎回 思った。
meaning "Every time I encountered a difference between life in マンション and life in a 一戸建てor アパート, I felt that マンション was better."
ずっと is sometimes used for something that repeats, also for something instantaneous but frequent.
similar to 頻繁に, 常に, 毎回。
Also, unlike 思った, using 思っていた near a clear context of time without information on how you think now usually means you dont think so anymore, you changed your mind. Especially when you're talking about a point of view. In such situation, if you still think the same way, you should say it explicitly.