What should I spend my hard earned coins on?

Dantheman,
I did max out my mon storage and items bag. It is a game changer.
I stopped buying lucky eggs at level 15.
I've never bought lures/incense.
I do buy incubators.
All with coin earned from gyms.

Storage.

You need it. Even if you are in a small town it would be best to sit, camp out a pokestop and stock up on items you most use and don't have to worry about running out.

It's all about that item storage, my man

i like incubaters personally.

Item storage> Pokémon Storage> Incubators> Everything else is how I value the items currently purchasable.

I might be ridiculous, but I like saving coins. We've seen that some items are occasionally discounted, and there's always the chance that some items (perhaps master balls or things useful for acquiring legendaries) will become available in the future.

So, except when there are events, I spend only so that I never need to put a 10k in my infinite incubator.

Having storages maxed is nice too, depending on what you're into. I doubt I would have held onto my three Shadow Claw/Ball Gengars if I had had more strictly limited mon space before Gen2 dropped.

1) Max storage
2) Incubators for 10k eggs
3) Maybe eggs when they are on sale if you care about leveling up but after level 30 it doesn't matter so much so this would be low priority

You don't sound interested in levelling up, so lucky eggs aren't required. Rethink this though if you want your Pokemon to reach higher CP (through Power Up).

You don't sound interested in collecting multiple varieties (movesets, IV, gender etc.) of the same mon or having a living dex either, so additional Pokemon storage not required. Personally, maxing storage and bagspace enhanced my game experience.

You also ruled out additional bag space. What are your goals in the game now?

If you're interested in getting better IV mon, incubators seem like the best investment for you.