Learn English – “It will depend on what I [will] see”

"I have to see you sometime" means any time in the future, starting now. "No time like now" is a valid reply to that, suggesting the "sometime" to be very, very soon.

“I am going to have to see you sometime” puts an additional time delay during which you won't have to see that person - such time will come eventually, but the nearest future is exempt.

As for will/going to, I suggest previous answers distinguishing the two.

The rule you have been told has some validity, but is too general.

English speakers don't use a will with simple future meaning after if:

If the plan succeeds, I will come.

not

*If the plan will succeed, I will come.

But will can also convey intention or willingness, so with an animate subject (especially second person) will can work

If you will come, I will talk to you

meaning something like if you are willing to come.

So

If you jump, I'll catch you.

is normal, as is

If you fall, I'll catch you.

But while

*If you will fall, I'll catch you.

doesn't make sense,

If you will jump, I'll catch you.

can make sense, with the special meaning of if you are willing to jump - it's an invitation, or a dare.