Are ‘fekr konam’ فکرکنم and ‘fekr mikonam’ فکرمی‌کنم used the same way?

When used to give your opinion, as in ‘I think’ or ‘I believe’, they are interchangeable: Fekr konam/Fekr mikonam hanuz xabar-o nashnide فکرکنم / فکر می‌کنم هنوز خبرو نشنیده ‘I think s/he hasn’t heard the news yet’.

However, when the usage of a certain tense is of significance, they are not interchangeable: A: Do you think it’s a good idea? B: Nemidunam, baayad darbaarash fekr konam نمی‌دونم، باید درباره اش فکرکنم ‘I don’t know; I have to think about it’. Here, fekr mikonam فکرمی‌کنم is not possible because after baayad باید we need the subjunctive.

OR

C: Do you think it’s a good idea? D: Nemidunam, darbaarash fekr mikonam behet migam درباره اش فکرمی‌کنم بهت می‌گم ‘I don’t know; I’ll think about it and let you know’. Here, fekr konam فکرکنم is not possible because there’s nothing that requires the subjunctive. The simple present simply denotes future time.