Well; I'm not a Zilean main exactly. He's just by far my most played mage; I'd say I play him at least once in every twenty or so games on average, and I have years of experience playing him. Some weeks I play nothing but him, just because he's so much fun for me.

I've mained Cho'Gath top since LoL was new, up until I got into Season 9. He's easy mode for me at this point; I never lose a game with him unless we lose in the first 15 minutes, because he starts getting into 1v5 mode around 20 minutes even when things are going badly. I wanted something very new so I made the switch to maining Yuumi support and am having a grand time with her. She's way better on the ground than people think she is at carrying games as either the engage tank or as a tanky extra ADC than can heal itself or just become outright untargetable when things get too dicey. She can still do 80%+ of what AFK Yuumis do; heal the team, ride a fed teammate, catch targets with her ult, etc etc. I laugh at myself because I went from the champion with the most health to the one with the least, and I ended up making her into a little semi-unkillable tank because that's how I play.

I use Hextech Gunblade as third item on Zilean, when I want an midgame extra tool to catch targets for my team. But usually, I am good enough with my bomb placement to not need it. It has that short 40s cooldown active slow, AP for the bombs, and AD for the auto attacks... pretty ideal. When I know that I'll be building it for sure, I have gone for Ingenious Hunter over Ultimate Hunter to get that 40s cooldown down to 24s. It serves much the same function as your build. GLP could be an option too; I might try that one of these days. It would be better bomb AP and could slow multiple targets at once. I've never opted for it on Zilean before.

I'm not a fan of the Glacial Augment rune. The item actives are doing almost all the work; the rune itself does almost nothing on top of what would be happening anyway. There haven't been any champions running it where the build really impressed me. You might try a few games with the same item build, but running different runes. I will say that Glacial Augment looks reasonable on a ranged jungler that lacks cc, to help gap close onto the target. That's about the only spot I see for it where it seems to be doing a lot to actually change the outcome of engagements.

If you like peeling targets for your team (from range), you might try Frozen Mallet + Archangel's Staff as your build core. You can then, if you want, go into Warmog's as third item and have so much health/regen the enemy team won't particularly want to target you first. This lets you vanguard a bit with your bombs and your Frozen Mallet slow, to catch targets. (The last two items there are Deathcap and Void Staff; still plenty of AP in the build.) The only issue with it is that there's no room left for Nashtor's Tooth (or a support item). But it does work and was something I did sometimes when we were winning but needed to have sustain while across the map sieging.