Without multiclassing or feats, you are restricted to your own class's spell list. The Circle of Spores, like all druid circles, gets a list of additional circle spells added to the druid spell list for them, but those are the only spells outside the normal druid spell list that you can access without multiclassing or feats.
On the other hand, you can find magical items that allow you to cast additional spells, e.g. a wand of fireballs, which any spellcaster can attune to and use to cast fireball. That will use the wand's charges and DC instead of your spell slots and spell DC, however; different items will have different rules on that.
Spell lists are matched to classes specifically to create class flavor and "branding," so to speak, and the druid spell list reflects that -- and doesn't necessarily have the same sort of offensive spells that other classes might have. Call lightning, ice storm, and wall of fire are all examples of offensive druid spells that can be used at range and have a Dex save instead of a Con save. (Notably, all of those also do area-of-effect damage.)