It's already been clarified by Crawford. You can find it yourself or run it as you wish. Crawford made it clear that all the rules of spellcasting apply to items unless the item specifies otherwise including concentration, use of the Cast a Spell action, and no bonus action spells other than cantrips. The wand of fireballs specifically states in the DMG that you "cast a fireball." If you want to run it some other way, have at it. I'm merely advising the OP on the rules in the DMG and the clarification by Crawford.
I haven't seen that clarification. Hmm. Looking, the slot and component stuff are stated on page 141; "the spell is cast at the lowest possible spell level, doesn't expend any of the user's spell slots, and requires no components, unless the item's description says otherwise". So I guess I'm willing to buy that interpretation. They do contrast with the way that potions bypass casting to merely confer a spell's effects.
So, for a contrasting case, look at the brazier of commanding fire elementals, which lets you "use an action to speak the brazier's command word and summon a fire elemental, as if you had cast the conjure elemental spell." That clearly is NOT using the cast-a-spell action, I think.
That said, now I'm curious about wording. Look at the cape of the mountebank. "While wearing it, you can use it to cast the dimension door spell as an action." Why does it need to specify "as an action" here? Also, I note that some wands specify a fixed save DC, while staves usually use the wielder's proficiency and casting ability. Hmm.
Okay, that brings us to a lovely hypothetical. So, say you have a wand of wonder. You invoke it. Some but not all of its powers are "you cast...", and the wording clearly distinguishes between "if the effect causes you to cast a spell from the wand" and the non-spell-casting versions. So. You have invoked the wand. You roll on the table. At this point,
if you got a "you cast..." effect, you cannot use a bonus action to cast a spell with a one-bonus-action casting time. But if you didn't, you can.
So what happens if you cast a bonus-action spell, then attempt to invoke the wand of wonder? It's
not known until you roll on the table whether or not it will be the "cast a spell" action. If it's not, you should be fine. If it is, what happens? Nothing? The spell fizzles?