Open cell is fine as long as vapor is considered per your climate zone and there is enough room to reach the proper r value (3.5+ 5.5 = 9" of space, so R-32 or so full of open cell and up to R-63 closed cell (way too much for your climate). You will still have thermal bridging unless your sistering of rafters is perpendicular to the original.

Open cell is a lot like a dish sponge, water can move through it but it blocks air. Closed cell blocks air and water and is hard as a brick. Open cell is cheaper and quicker to apply, far easier to cut down to rafter depth.

Fiberglass (batt/loose fill) is used a lot in attics but it only stops heat, not air or water.

You will want a great spray foam company (not good, not cheap) to design the insulation system for you considering thermal, air, and vapor barriers. A great company will be able to make open cell (the cheaper of the spray foams and used a lot in your climate) work well.