Short end of everything, just make him ability power based. Making him build weird stuff to fulfill the exactly same thing just makes it a nightmare to try to balance him. With ability power ratios allows him to be this squishy team-reliant utility assassin and not be oppressive and can tune him exactly how you want to tune him and he can build normal support items.

A part of this is removing the defensive stat restrictions to give him normal health and defenses and can build them normally as well. To encourage the squishy build you intentionally give him lower base values and scale well with ability power (the less base health makes up for incidental health from AP items which comes out to about how much health - roughly - to what he gets now more or less).

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Going to give some examples below of what it would look like if he has ability power for his scaling. The actual kit would be unchanged outside of a few small changes and additions and values based on where his lethality and attack damage values are set at (they are all bonus attack damage values so pretty easy conversion rate).

Again, not played differently, just a lot more streamed line and easier to build for - also opens up player customization and expression for builds doing it this way which is much better game design allowing the player to choose what they want to do on the champion. With the benefit of taking Ardent Censer if he takes the CC to healing mastery to proc Ardent passive this way as well.

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Gift of the Drowned Ones (Passive):

Pyke stores 30% - 80% of the damage he takes from enemy champions as Grey Health, up to 80 (+500%AP) health.

When Pyke is unseen by enemies, he rapidly consumes his Grey Health to heal himself.

Additionally, Pyke deals up to 25 (+100%AP) bonus physical damage on his basic attacks based on the target's missing health.

Bone Skewer (Q):

Damage: 60/90/120/150/180 (+70%AP) physical damage. 115% damage if he taps it.

Ghostwater Dive (W):

Movement Speed: 40% (+10% per 100AP) decaying over the duration.

Phantom Undertow (E):

Stun Duration: 1.1 (+0.1 per 100AP) seconds Damage: 90/115/140/165/190(+90%AP) physical damage.

Death From Below (R):

Damage: 250/400/550(+100%AP) physical damage. True damage when executing below this health threshold.

(Does not make sense for this to scale with level instead of ultimate rank, they both fulfill similar aspects regardless, it is just more clear for the player and their opponent to know how much the ultimate does based on ability rank rather than champion level since players know of the levels 6, 11, and 16 power spikes and should be consistent with all ultimates in the game.)

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Exact same gameplay, easier to tune and balance doing it this way and opens up player choice to how they want to build Pyke without some weird ass build. And by having damage based on ability power, you can better tune how much damage he does for his basic attacks this way, or he gives up ability strength in the process by going attack damage like how the system was intended to be set up in the first place.