How powerful is this sword? [D&D 5e]

[Scarey Nerd]

I asked about this in a thread in the wrong forum, so I'm reposting here with some adjustments. One of the players in my current game is playing a Half-Elf Bladesinger, whose mother was a Bladesinger before her, and the player has her sword. I'm confident that she won't want to put that sword away at any point, but I want it to get more powerful as she levels up in line with other players receiving magic items as well. I tend to give out magic items with decent frequency, though I try my hardest to stick to the rarity guidelines from the DMG. Anyway, here it is, if it sounds far too powerful then I'll try tweaking it down, or if it sounds like it wouldn't be that useful I'll try scaling it up.

Song Blade
Weapon (any sword), very rare (requires attunement by an Elf or Half-Elf Wizard with the Bladesinger Arcane Tradition

Members of the Ael’Tahlaer, the Knights of the Singing Blade, wield these swords. Dating back to the founding of the ancient order of mages and the gift of the Bladesong from Solwyn [Elven goddess of magic and music in my world] herself, Bladesingers have passed down these enchanted weapons from master to student for generations. Each Bladesinger sword, no matter if it is a Longsword, Shortsword, Greatsword etc., bears the crest of the Ael’Tahlaer, somehow never gets dirty, and weighs half what the weapon would normally weigh.
When the bearer is a 5th level Bladesinger, the sword receives a +1 bonus to attack and damage rolls; at 11th level it receives a +2 bonus; at 17th level it receives a +3 bonus; and at 20th level, once per year when Solwyn’s constellation is directly above them, the bearer may perform a ritual that requires cutting their palm and gripping the hilt, and expending a 5th level spell slot – the bearer can then speak with the deceased previous bearers of the sword as per the Commune spell, with the restriction of asking only one question. If the bearer expends a higher level spell slot, for every level above 5th an extra question can be asked.

[Ninja_Prawn]

Well, it's not overpowered. So there's that.

Personally, I don't think scaling enchantments gel with the rarity rules, to the extent that I'd rather make it a unique artefact, divorced from that whole system.

Even if you don't do that, you'll have to give this one a bit more oomph. Say you give the paladin a Sun Blade at level 11. That's entirely reasonable. This Song Blade will look distinctly ordinary in comparison. So I'd pack it full of powers - especially ones that provide now tactical options instead of 'bigger numbers' - but not give the baldesinger anything else. The others can have fancy boots and cloaks and rings, but everything the baldesinger does comes through this blade.

And the once/year Commune thing is weak and arbitrary. I'd cut it from the weapon and have it just happen when it needs to.

[WarrentheHero]

You could consider making this similar in nature to the Bardic Instruments. It could have a small host of spells, especially utility spells. That way you give the character a magic sword, and allow them to prepare more spells that they actually want instead of Fly because maybe there's a cliff.