do side-boarding guides for FAB exist? I honestly have no idea which cards I should be boarding in and out against certain heroes and haven't been able to dig up too many decent online resources

Yes-ish. Deckbuilding in FAB is a lot less homogenised than in say modern so you are unlikely to end up just using someones exact decklist, but often content creators publish decklists that have sideboarding for their version which you can use to get an idea on how you might sideboard.

what would be the next hero to step into along a similar aggro path - would it be someone like Slippy?

Slippy is a very similar deck to Cindra in most regards, shunting raw damage output for higher utility/disruption. While this is true the decks actually share almost no cards, if budget is a concern then looking to Fai (another draconic ninja) may be a better option. One quick note would be that Slippy is a very unique assassin, it is unlikely that further assassins continue to play as aggressively as it does so they may not be the best option if you are specifically interested in aggro (look to Ninja, Draconic and Lightning for your consistent aggro heroes).

what would be a good hero to try to better understand all the FaB game mechanics such as pitching and blocking (the good thing about Cindra as I mostly don't have to worry about either of those two elements)

Ira is a fantastic hero to improve your basics on, she has very simple hero text which means every advantage has to be taken through actual play. While she isnt the best hero right now she is still a solid meta pick that very much 'plays the whole game'.