Disparity between Prepared and Spontaneous Multiclass Spellcasting Archetypes

[First World Bard]

We learned that Signature Spells (spontaneous heightening) are a class feature for Bards/Sorcs here. Given that it's not baked into casting for those classes, it seems like spontaneous multiclass casters are at a disadvantage compared to their prepared counterparts. Basic/Expert/Master multiclass casting get you 1 slot per spell level. If you're a prepared caster, you pick whatever you'd like each day for those slots, heightening as appropriate. If you're a spontaneous multiclass caster, you have 1 spell known and 1 spell slot per spell level, which doesn't seem all that spontaneous. Maybe you can use a higher level spell slot to cast one of your lower level spells known? I'd assume you can do that, but don't actually know for sure.
Things get better for the multiclass spontaneous caster if they can pick up their Breadth feat, which gets them an extra spell known and an extra spell slot for all but their highest two slots. But yeah, I'm a little bummed that they don't seem to get access to Signature Spells.

[First World Bard]

Assuming this is working as intended for the designers, I'd be interested in some way for a multiclass spontaneous caster to access Signature Spells. I don't think it's worth a Class feat, maybe a Skill feat though. I'm just workshopping here, but what do people think of:
Bardic Signature Spell {Archetype}{Skill}
Prerequisites:Basic Bardic Spellcasting, Expert in Occultism
You gain a single Signature Spell.
Special:If you are a master in Occultism, you may take this feat a second time. If you are legendary in Occultism, you may take it a third time. You must choose a spell of a different level than your previous Signature Spells.

[Siro]

Not a bad idea, I like it as it does add the spontaneous to the spell casting, and from an RP standpoint with deeper knowledge (ie in prof) and more specialized knowldge(ie skill feat) let’s you do more stuff with your side spellcasting hobby. ( would be an interesting idea to explore in it of itself, even outside the multiclassing aspect) My two concerns would be 1) Spontaneous multiclassers would still need to go through an extra step to become Spontaneous, and 2) lesser concern, but it would still take a couple of levels at lower levels, before the ‘Spontaneous’ came online.