I realize I didn't answer your question.

6061,6063,6082 will all work. Your pole is probably 6063 T6 since that is what Forespar and some others makes their poles out of.

Don't buy cheap al tubing. Buy from a marine distributor or an established metal distributor and be sure to tell them what you are planning to do with it.

If you are doing this because you just don't want to pay NACRA prices, 6063 is a good choice.

If you are doing this because your pole is not strong enough Try 6082,6061 or even 7075.

Try to look at a pole right after it breaks. If there are cracks with white powder near the break, I would definitely treat the inside of the pole to prevent corrosion. The surface and the anodize is cracking allowing salt to corrode the al. It would have probably broken without the corrosion but the corrosion makes it break faster

If the poles always break at the same place, try splicing the pole. Use a 2 ft tube whose OD is the same as your poles ID but twice the wall thickness of the pole. If the pole breaks again somewhere else, it was cheap.