Keeping in mind that my main goal is to make it so that the elements present in the setting on day 1 are viable on day 750, and that PCs do not "outgrow" their position in the world or break the previously established boundaries of the setting, what kinds of rewards in play do you think would keep players happy?
This is not broadly answerable. Each player has their own desires, and not everyone will feel engaged by the same set of rewards. YOu need to ask or otherwise know your players to really answer the question for your table.
But, let us liken this to... giving gifts to adults. Adults are not usually in a state where they actually need (or even have room in their homes) for a lot of *stuff*. Once you've set up house for a while, you eventually accumulate what you need and... then what? What do you get them as a gift?
There are two basic answers - you give experiences (like, tickets to a show or concert) or consumables.
In D&D, consumables are easy - charged magic items, potions, scrolls, money, and so on.
What kind of character development and "advancement" can exist aside from level gain to reinforce the goal while still giving players a sense of accomplishment and growth?
D&D's class-based system is great for many things. It isn't great for this, unfortunately. My first approach to running a game that is based on character development rather than character power advancement would be to use a system that works in those terms. FATE-based games are good at this. Some Cortex+ games are.
In D&D, a character is defined in terms of their stats, race, and class, from which all their abilities are derived.
In FATE games, as an example, a character is generally defined in terms of Skill points and Aspects (which are verbal descriptors that can be used for bonuses). In FATE-games, you can have a game in which characters don't earn new skill points quickly (or at all), but who can shuffle their skill points around over time and changing their Aspects in response to events in play. So, you can start with a wild, knife-wielding gang thug, and end with a disciplined gun-toting mob hit man, changing the character's mechanics over time, without raising the character's net power level.
This is difficult to do in D&D, as we don't have a good handle on the atomic elements of the character's abilities. We don't have a way to transition a character from one sub-class to another, or from one class to another.