As you said yourself, this has far more to do with Favours and Allies than with Potions.
How do Debt and Potion feel similar? They are like the antithesis of each other. You can always buy Debt cards whereas you can rarely buy Potion cards. That is why your Vineyard variant is beyond broken.
Huh, Golem for $5? You are aware that Golem is stronger than Lost City, which is a $5 with a nerf, right?
Golem is not necessarily stronger than Lost City, because it can't play any cards from your hand, and is more difficult to chain in most games, especially if you draw it late during your turn. I will say that my statement about how it is a weak $5 was wrong. It's a strong $5, but not as strong as to always being strictly better than Lost City. In a kingdom with decent to strong trashing and a non-terminal draw alternative, Golem is easily outclassed and obsolete (especially with its current cost).
Sure, that is why Golem costs $4P. It does not get more elementary than the crude „if you implement a Potion as a normal card, it has to cost at least $2 more without the Potion“. This is crude because it does not include all the subtleties about Potions and it is a lower benchmark.
So again, why do you do Golem as a $5? If it works with ordinary costs, it has to be at least a $6 or include some nerf.
Beyond this elementary error, you seem to oblivious to the reasons Golem is so expensive. Digging for Actions is on average stronger than drawing.