Exterior and Interior Roof Insulation – Potential problems with a two step delayed project?

I have a one and a half story in zone 4.  Pretty typical with these constructions, I have issues with ice damming.  The house has a vented shingled roof with various amounts of insulation in the ceiling and knee walls.  Air can travel from soffit openings to ridge vents.  Next spring I will be getting a roof replacement and want to take the opportunity to make improvements.

Ideally I would get all the required R value in insulation on the outside of the roof sheathing and seal attic venting.  Given the climate and my budget, this is probably unlikely to happen.

My next option is the to do a combination of exterior and interior insulation and similarly seal attic venting.  Interior insulation is going to require removal of all the second story ceilings, which is a bigger project than I can afford to do immediately.  When it happens, I will have about 6in to work with between the rafters.

My big question then – Am I okay doing exterior insulation and waiting on both interior insulation and vent sealing?   Am I introducing any new problems in the mean time?  This will likely be on the order of a few years – at that time I can redo the interior, add insulation to the proper R value, and seal attic venting.

In the meantime I don’t really get any energy savings, but I should reduce the chance of ice dams and heat transfer through the shingles.  I’m aiming for better than it currently is and working in the right direction – without causing any new issues.

Thank you for your thoughts!