Upgrade your iPhone to iOS 9.
Upgrade your iPhone to iOS 9.
Keep your jailbreak intact. Yes, you can still back up Cydia tweaks and apps.
I understand the issue. Upgrading to iOS 9 instead of 10 isn't possible through standard methods.
Sorry to inform you, but you can't upgrade to iOS 9. The latest version you can upgrade to is 10.3.1, which might become jailbreakable soon, but it's uncertain. If you choose to proceed, act fast because this release could be signed off in the coming days. Apple typically removes signatures a few weeks after a new version drops. To work around this, you'd need an unsigned blob of the desired version, which means your device must have that older, signed OS installed. I don't fully understand their reasoning, but it seems like a difficult situation with no clear solution.