I don't see why you wouldn't level your Ult. It is the only skill with a CD that gets lowered as you level it. If you are under the impression that your passive will be better because your Q will have a higher CD, keep in mind that all your other skills benefit from CDR as well at an equal percentage. This means that his passive lowers your CDs by the same percentage per spellcast, regardless of your CDR stat. Also, levels in your ultimate increase the duration of your passive, which means more spells that you can use, which will lower your CDs to the point where your Ultimate is basically always up (at lvl 3 R and Passive). I understand that if you wanted more passive shields, you would have to wait longer, but when your passive is down, you still have to wait for FULL CDs to proc your passive again as you don't have the benefit of your passive any longer. By using your strategy, you are greatly mitigating your own damage output for not much gain (an extra 400 health shield in a teamfight that lasts longer than 4 + QEWQ..wait..E (passive proc) at least 11 seconds bc you don't have CDR). Effectively, with maxed Ult, you can proc your passive after it is on CD because your Ult will be back up which will give you another spell to use to help stack your passive meaning you only have to wait for your second Q after your passive is up rather than waiting for your second E. So we have 6 seconds on passive + QWERQ (passive proc) = 9-10 seconds total before passive is back up after it procs the first time.
Math time 40% CDR on max Ult = 24 second cooldown. Q's CD=2.4 seconds. W's = 8.4 seconds and E=4.2 seconds. Ryze's Passive reduces CD's based on Q's CD. In 6 seconds during passive Ryze can WQEQWQEQ .4 seconds for cast times spacing = 3.2 seconds + animations giving you plenty of time to pull off this combo (or even more). This will reduce your R cooldown by 2.4 x 8 = 19.2 seconds + 6 seconds of passive meaning your Ult would be up again, even if you only used 6-7 abilities during your passive even though more abilities are possible. Stacking your passive again would simply involve using your full rotation of spells including your Ult which shouldn't take too long as your Q has a 2.4 second CD. In a maximum of 10 seconds (given you had to wait a little for another CD) you can have your passive proc after the first proc with max CDR, only having 3-4 seconds of downtime.
Comparing that to only leveling your ult once. Even with max CDR (From FH and IBG--because I'm guessing that's what you're building), your Ult will have a 48 second CD, 2x what it would be otherwise. You would activate your Passive while your Ult was active, and you would get less spells off before your passive ran out, AND your ultimate would still be on CD. Meaning you would have to use your spells until you got to your second E rather than your second Q, nearly 2 more seconds where you would be doing nothing besides AAing (good thing you can finally use your IBG :P). In total you would have the 4 seconds to have your passive up + EQW..Q..E 6 seconds for your other abilities to proc your passive and again you will have your Passive back up in 10 seconds after your passive was used the first time. The only difference is that now you don't benefit from your passive as much as if you leveled your Ult.
At max lvl ult and passive, you have MORE lockdown in a teamfight than your method as you can get 3 Ws off during
your passive (last one is when your passive expires). For the love of God, don't build IBG on Ryze. He has 106 AD at lvl 18, one of the lowest in the game; the 1.25 AD multiplier means he will deal 132 damage with the spellblade proc and in the midst of a team fight, Ryze doesn't have much time to AA. I can see the stats IBG gives as being "tempting", but at 2900 gold there are so many better items both defensively and offensively. Randuin's is cheaper and gives more effective health. (500 mana = 40 points to your shield) Rylai's (though not recommended) is the same price and gives you more damage with the addition of a slow on each spell cast. Deathcap and Luden's are only a few hundred gold more and offer WAY more damage. Lichbane would even be better for Ryze in terms of spell blade effects even though you won't be using your autos much as Ryze when he is running around and throwing spells. If you want another item that gives effective health and lots of damage, get another RoA as it will not be wasted on Ryze. If you need Armor, Thornmail is one of the best in the game against Double AD comps and benefits from Penetration and MR shred which Ryze can build a lot of. Guardian Angel is even an option to give you Armor and MR.
I'm glad you are looking into gimmicks that can be used for champions to make them as strong as possible, but Ryze benefits a LOT from leveling his Ultimate. The downtime of his shield is comparable at max CDR whether or not you level his Ultimate, and it's better to level his Ulti for the added CDR so you don't have to dedicate items to that stat. I hope you the best of luck out there. You are correct in that Ryze will be best when your team has a lot of Magic damage and when the enemy has a lot of sustained damage over burst damage. Ryze is going to do very well against tanks as well because he can output sustained damage and shred lot's of MR with his E. In my mind, he might be broken in his current state as his Ult basically has no downtime and his passive can be up almost indefinitely in the late game. He might have some nerfs coming in, particularly for his W, which can be used to lockdown targets very hard when his passive is up. Thank you for reading and have fun out there. Peace.
