I'm not sure why I didn't think of this sooner

The nail in the coffin for this season was the support mastery changes. And his early game can be hard to come back from if he gets ahead. Solution:

His Heartseeker Strike passive restores 2/4/6/8/10 mana every time it activates.

This change would give his expected mana restoration to be between 6-12/12-24/18-36/24-48/30-60 mana per 30 seconds depending on how well you cs, which is 0.2-0.4/0.4-0.8/06-1.2/0.8-1.6/1.0-2.0 mps. Even with perfect cs, you don't cover the 0.6 mps he lost from the support mastery changes (and he was already hurting a lot from BC's recipe change and the meta shifting to a very tanky top lane). You need decent cs and two points early on to get that mana back.

The problem with just buffing his base mana regen would be that his power in Season 4 was absurd top lane. While I'd like to be able to spear people to the dirt more easily even if they are tankier, I do realize that spears are very oppressive. In comes the change. In order to have a net regen in mana, you have to be in melee range and auto a minion instead of Qing them, which slightly reduces his average safety in exchange for more mana at the player's discretion.

Additionally, Q is the oppressive thing, not E. Rank 1 Q hits well until about lvl 5. His power spike only shines with rank 2 and 3 Qs and possibly a brutalizer. Putting ranks into E does increase damage but make him easier to fight in melee.

Allowing player to opt into more mana for the ability to do their favorite thing, chuck spears, while simultaneously making spears indirectly less powerful early on AND giving some more opportunities to abuse Pantheon's single gear of going forward for the enemy seems to be a plus in my book.