One of the bard's themes is they can learn a bit of anything. It's the jack of all trades concept where they can learn several things reasonably well but don't master any of them. We can see this in their skills, spells, and subclasses.
If we compare the bard to the rogue the rogue is obviously better at skill checks. The rogue has one more proficient skill, gains expertise faster, and has reliable talent. Bards are good at skills but rogues are better. Rogues also have better combat abilities.
If we compare bards to any martial oriented class then bards fall very short. Poor weapons, poor armor, no combat benefits. Bards can use a subclass to gain more martial prowess. They'll still never soak up damage like a fighter or barbarian and consistent damage is limited by spells. They can have a martial option but they aren't the best at it. This is also a reason I wouldn't give the, free weapon mastery. Bards don't need that boost to be better at it when they're not meant to be.
Bards improve combat through bardic inspiration instead and use skills and combat inspiration as an alternative to things like channel divinity or wild shape and the better armor options of their fellow support casters in the cleric and druid. One of the reasons for bards to be strong spellcasters is as a viable alternative to clerics and druids.
If we compare bards to other spellcasters at spellcasting bards still fall short. Clerics and druids have a lot of bonus spells prepared , access to their full lists with a rest, and magical benefits in channel divinity, blessed strikes, and divine intervention; or wild shape and elemental fury. Divine order or primal order give better armor and weapons to the base class or skill benefits and more cantrips than bards. Wizards have arcane recovery, ritual adept, a much better spell prep mechanic, and memorize spell to be obviously better than bards trying to be wizards. Sorcerers have innate sorcerer, font of magic, and metamagic to be obviously better than bards as spellcasters.
Warlocks gains spells just as fast, also have a lot of bonus spells, and a lot of at-will power in the invocations. Bards could have done something similar but would need a lot more development of the bard songs as an alternative to invocations. I see that as pointless if the songs do what spells would do anyway, but if someone wants to use mystic arcanum style magical secrets, songs instead of invocations, and bardic magic instead of pact magic it could work in their house rules. What I don't like is trying to tell everyone that's the only way bards should be designed. ;-)
Bards are still in the 3.5 mode where almost everything they can do is done better by another class. Now they're designed well enough they're still worth playing because they're more similar to druids and clerics.
Thematically, bards are powerful magicians who rely on wit and skill as well as magic. They support and inspire the party.
The powerful magic part is thematic differently through spell selection. Spell preparation is still a huge limiting factor on bards. The bard spell list already consists of many cleric, druid, and wizard spells. A bard who takes a lot of shared spells from one of those lists is already leaning into one of those styles at 10th level. A bard who tries to split them up is already dabbling in each.
The broad access is always there. Magical secrets is less like adding more, and more like removing restrictions to the full lists. Which is then handled exactly the same way where the spell preparation mechanic is a limiting factor, the bard is either dabbling or focusing on a spell list to match the spell style of the bard, and the bard is failing to be as good a spellcaster as a cleric, druid, sorcerer, or wizard.
We can create our own concept of the theme. Bards are a very customizable class. I like that. They start as an inspiring
skilled magician and we build from there based on our character concept.
Of course not.
I'm on board with grandmaster features accessed by barbarians and fighters.
