My subs been expired since January, I have an ilevel of 873. I want to come back and my three main goals are getting the unstoppable force title for honor, flying, and to complete the mage tower challenge for the flail.

Do I have time to do these things (Especially to get the legendaries and ilvl needed to successfully complete this challenge) by the end of the expac if I start now? Do I need to start now or can I wait further?

Should be fairly easy to complete these prior to the end of legion.
I have a lvl 101 prot pally and its ilvl 875, so for a 110 to get to ilvl 900 and complete your goals wouldnt take too much time, specially if you can find someone who's already ran out of things to do and has flying and can burn you around the map.
The MT Challenge is hard as fuck but if you watch a couple videos and pay attention it should go down after a few tries.

Get started now. No telling how long it'd take for you to get your legendaries that you need, but you can easily gear up quickly, and a new raid just released this week so you can outgear the challenge easily.

I logged on for about 2 hours before asking blizzard to freeze my time. Definitely an odd complaint about an mmo, but there is just too much stuff to do to gear up, get my ak/ap in a better spot, maintain gold for tokens, wait for mage tower every week, etc and I simply don't have the time to be on this game for the amount it would take to figure everything out and get to end-game. Not least of which for me personally, they have absolutely gutted prot paladin in instanced pvp which is killing my incentive to play - I guess they didn't like having utility class in competitive levels? Who knows maybe next expansion it will be good again. World pvp could be ok but I am at such a low ap/ak level, item level, that the same problems exist. Who knows maybe next expansion it will be good again.

I'm a holy paladin 865 gear score

Pretty lit so far.

Did a mythic +5 today with my cousin and it was crazy lol

Completed mage tower on guardian druid with some of the worst legendaries (no wrist/chest/pants).

Same could be done on pally if you are a decent player. Next tier mage tower will be a joke if it isn't buffed.