Implant deflated and lift
Deflating the implants can help decide if a lift is needed, but removing them through the armpit culd be difficult if you wait, and if you need a lift it would not make sense to use that approach.
Don't think how long you wait matters much
I can't see that waiting 1-2 months versus 4-5 months matters a lot for the scenario you describe. Once the implants are deflated, a decision as to what is needed next can be made between you and your surgeon. If a lift is needed, it would not make sense to go back through the axilla.
being able to deflate or know a rupture In saline implants is a big benefit to them. Deflating them prior to removal is a good plan I know if you need a lift. Most people who remove their implants need a lift, but that depends in the implant size and your breast size and shape. I always tell my patients that the longer you can wait the better. If you can tolerate 3 to 6 months than that gives your body the best chance to know what it will be like. The big problem is that you may develop fluid outside your implant which can as volume (Seroma). That can complicate the situation and is not an uncommon thing. I often perform fat transfer (liposuction in one area and transferring that fat to the beast without an incision.) to increase breast volume after removal of implants for a natural look and to avoid a breast lift.