Honestly, the only thing worth getting from the store are storage upgrades. I got something like 250 Poké Balls while wandering around downtown today and having extra storage and just ditching other (healing) items you don't need as much helps. I don't really spend much time in Pokémon Gyms aside from just getting a spot to get a little bit of in-game cash.
Basically prioritizing what you want to do helps (catching them all, leveling, fighting at Gyms, etc), and while you CAN level up faster via various tactics, it's probably better to just go about things normally if you just want to collect them.
Insofar as the massive XP curve, Ingress' XP literally doubled every level. It was set to cap at 8, but eventually they opened it up to 16 and people STILL hit it. Leveling is all to give even hardcore players a REALLY high end goal, and it meant that most folks won't get there for a long time, which helps to stretch things out while they build up other big events and things for the game, rather than having the player base run wild with level imbalancing, etc. and running out of things to do.
After my wandering today, I used a Lucky Egg and finally did a bunch of Evolutions I'd been saving and it netted me 29,000 XP in 14 mins, and pushed me up to 21. Ultra Balls theoretically make things easier, but as soon as I hit Lv20 today, I ran into a CP 1053 Clefable. Razz Berry + Ultra Balls = all three broken and it ran away (devastating my excited little heart). Apparently Curve Balls help with your chances, so I've been practicing them and they actually seem to make things easier in almost every respect.
Managed to find a bunch of other cool Pokémon, and I'm now at 90/92 in my Pokédex (because of that Clefable and a 109 Onyx that took a Razz Berry + Great Ball and instantly ran earlier this week).
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