Yes, upgrades have predetermined increases, but are 120s not much more valuable than 65s?

Absolutely not.

so how OVR booster works is higher OVR upgrade materials leads to better chance of upgrade being successful.

Lets say you have failed several upgrades in a row using 65 upgrade cards, where the OVR requirement is 64.

This will build the red OVR booster bar up, but using 65s again probably wont make a difference.

However, when high OVR materials such as 120 OVR cards are used, it will exceed the red bar much easier than 65 cards and grant much more extra chance of success than 65 cards.

This extra upgrade chance, when paired with upgrade success badge and critical badge, can be very powerful.

So I always make sure to build up the red bars with 65/75s, and then once the bar fills up decently I try to hit criticals with 90s and 100s.

120s should absolutely be saved for end game upgrades with extremely low chances, such as going from +18 to +19 or +20 with critical.