Alright, I’m seeing your reasoning a little more clearly. I disagree with it and I’ll explain why.
The Staff of Power. Is a staff, but makes no mention of being allowed to be used as a focus, it clearly can be, it can also be used as a magic quarterstaff. In 30 years of playing I’ve never seen a DM rule that a Wizard staff can’t be used to just cudgel someone as a quarterstaff, though I think the Focus concept wasn’t formally introduced and normalized until 4th edition.
And yet the Staff focus has no listed damage or reach and costs 5gp while a regular quarterstaff which does have those mechanical rules is 2 sp.
Rules as written a spellfocus staff would be an improvised weapon and a regular quarterstaff can’t be used as a focus.
You have inferred that save DC bonuses are tied to focus items. I could be wrong, but I don’t believe the Rod of the Pact Keeper (the item I think parallel’s most closely) functions as an actual focus RAW because it doesn’t explicitly say it does and, as established RE: staves not every staff = staff for casting.
It’s not an unreasonable inference, but it is, in ly opinion, unnecessarily restrictive in light of the fact the problem can be ignored for 50gp, a negligible 1 time investment, whether you buy a component pouch or a Ruby of the War Mage (which explicitly turns an item into a focus but does not require a focus feature, merely the ability to cast spells to employ).
But we’re into a completely subjective and academic distinction at this point.
You feel it’s a hard rare but can see going the other way.
I feel it’s uncommon.
Rarity is only relevant to the player of there’s some kind of rarity limit tied to tier. As a +1 weapon for a primarily martial class it’s clearly tier 2, some DMs might balk at the thought of a level 5 pc having a Rare item.
Best of luck to the DM, I wouldn’t worry about the rarity, I think consensus is it isn’t overpowered.