I’m gonna assume it’s a typo and should be ~と思っています instead of ~て思っています
I covered volitional form + と思っています some time ago in class and the teacher differentiates it from つもり in that the latter is already more solidly planned, whereas と思っています depicts your intention but your planning might be a little less in place than with つもり。
Both mean the same thing, but the second one sounds stronger and could imply that you have your tickets booked and everything’s settled.
And then there’s volitional form + と思います which according to genki 2 implies that you just made that decision on the spot, as compared to ~と思っています in which you already planned to do something before the conversation takes place.
As for ~ことにする, my impression is that sentences with this phrase have an emphasis on the decision making itself rather than the decision. (E.g. I’ve decided to lose weight.)
That’s my take on this anyway, feel free to add to/correct whatever I wrote
On another note, Japanese is full of these kinds of delicate linguistic differences that after some time I just stopped looking up every single one and just try to read more and immerse myself in the language to get the “feel” of it. It’s also the beauty of Japanese