The wizard isn't broken.In case you missed the memo - I'm not happy with not being able to cast low level spells for damage - or let me rephrase that before someone pedantically attacks it - I'm not happy that low level spell slots are effectively worthless for damage spells as you level.
My proposed change fixes that problem without really impacting the martial / caster divide.
That fighters are worse than casters is not an excuse to avoid fixing things about the wizard.
My suggestion would be to upgrade spell slots themselves. Level 11, all 1st level spell slots are now 2nd level spell slots. Level 17 (or whenever cantrips gain their final level up) all 2nd level spell slots are 3rd level spell slots.