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How to express opinions in Swedish?

Tycker is the verb used for opinions. "Jag tycker att din engelska är bra."
Tror is the verb used for "believe", when you believe something is true (or false) but you're not completely sure. "Jag tror att du är bättre på kinesiska än vad jag är."
Tänker means "to think", as in the mental process, but is not often used when expressing opinions.

Your example is correct. If you replace "tycker" with "tror" it's not an opinion anymore, but a fact. You believe it's too late (for something). "Tror" is used when you've got some confidence in what you're saying, so if you're not sure you could use "kanske" instead.

Tänker is used a bit differently, it's main meaning is the mental process associated with thinking. It also has other uses such as "tänk om" (what if) and "jag tänker +verb" (I'm intending to +verb).

Another way you can express opinions is to use vilken/vilket or vad. Vilken/vilket is "what a +adjective +noun". "Vilket fint hus. Vilken gullig katt."
Vad is "how +adjective". You don't need a noun after this, only an adjective.
"Vad kul. Vad billigt."
You can also use "verkar" (seems), "för +adjective" (too +adjective), etc..

"Jag tänker fortsätta studera svenska för att jag tycker att det är ett fint språk och tror att det kan vara roligt att kunna det. Tänk om jag kunde tala flytande svenska, vad roligt det skulle vara."