How EXACTLY have you determined that the President is unable to be president; that he is in any way incapacitated? How is he “unstable”?
If we’re going by successes of policy, he is one of the most stable presidents we have had in living memory.
That you don’t like or even HATE his policies or his manner is NOT proof that he is unable to hold the office.
The 25th was written to replace a president that was incapacitated. It also reiterates that when a president is removed from office - for whatever reason - he is to be replaced in an order of succession.
But you cannot remove him with the 25th just because you say he is incabale of continuing in office.
If a cabinet and / or a Congress tried to remove a president who was not provably unable to hold office by incapacitation or was not provably insane, there would be a civil war, and with good reason. Millions of Americans will want to know what he did that proved he was incapacitated or insane or somehow unable to continue in office.
What would be the answer to them?
If you want to remove a president for what you believe to be cause, you have to impeach him or her. The 25th is not the vehicle for that.