Spell Slots. The mage has spell slots as a 9th level wizard: 4/3/3/3/1.
Spells Known. The mage knows the spells mentioned in this stat block. He may prepare up to 3 additional cantrips and 8 additional spells, which are not listed as they will rarely be used in simple combat and are up to the DM for anything more complex.
Spells Cast. The mage is assumed to have cast mage armor before combat (included in AC, can be dispelled).
Other Equipment and Weapons. The mage may have other weapons or mundane items. They are unlikely to be used in combat and do not affect CR.
Actions
Offensive Spellcasting. The mage casts one of the following spells, using Intelligence as the spellcasting ability (spell attack +6, save DC 14): cone of cold, fireball (will use 4th level slots first), Fire Bolt
Bonus Actions
Misty Step. The mage casts misty step with a 2nd level spell slot.
Reactions
Protective Magic. The mage casts Counterspell or Shield in response to the spell’s trigger, using an appropriate spell slot and the same DC as Offensive Spellcasting.
(ps: not being able to paste 2024 formatted stat blocks is annoying).
I find the way D&D 2024 does NPC spellcasters annoying... and not actually necessary. You could get the same simplicity as 2024 with something like
(ps: not being able to paste 2024 formatted stat blocks is annoying).