If you do all your dailies and tier missions, you'll usually have an extra week like this at the end of a tier, so no need to hold onto/care about them if you've hit 35.
Tier missions and the mission pass don't actually reset at exactly the same time though, we'll get new tier missions at 19:00 on the 4th but the current mission pass goes through the 5th at 04:00. So make sure that you hold off on next week's tier missions until the next mission pass hits, or you'll effectively waste the points and start the next pass at a disadvantage. But because of the "extra week" mentioned before, even if you do accidentally do that, you'll have enough leeway to hit 35 by the end of the next pass anyway.
wrt buying with SG + buffer period, I don't know exactly if it applies to SG as well, but the previous pass will remain accessible even after it's "over" so that you can pick up items from it. Possible that it lets you buy tiers too, but I have it maxed out so I can't check.
Thank you. This is what I was looking for.
Now I know to just complete my weekly missions, since the points won't help. The Tier is not as important to complete as the only reward from it worth getting. Is the points.