well it depends, i do think he is a little toxic but its because he manipulates heechan to gain his attention. Like he will do some things like get hurt in purpose or crazy things will happened to him just so heechan can be worried for him. Like he didn’t need to do all that because clearly Heechan likes him.
If you consider main story as final, then yes it's disappointing. His toxicity isn't something that harms Heechan but it's a pretty destructive game.
If we consider extras part of the story, then it's mentioned how Minjae felt more secured than before and kind of implying that he might not do it in the future.

For those who’ve read the novel or manhwa, can you spoil for me if Minjae will become toxic in the future because I don’t want to be blindsided