It doesn't work. Create or Show a build that could effectively challenge him.
It doesn't fail on your say so. It's obvious high level casters can have the spells I listed. It's obvious a DC 19 WIS save effect works on your build most of the time. It's obvious your passive perception and that of your minions is less than the minimum roll on any DEX 14 character with the invisibility spell and expertise stealth.
So on that note... half elf bard (lore) 20
Point buy 10 STR, 15 DEX, 13 CON, 10 INT, 10 WIS, 14 CHA becomes 10 STR, 16 DEX, 14 CON, 10 INT, 10 WIS, 16 CHA after race bonuses.
Feats -- shadow touched (silent image, invisibility +1 CHA), +2 CHA, resilient (WIS), metamagic (distant spell, subtle spell), +1 CHA/+1 WIS.
Final ability scores: 10 STR, 16 DEX, 14 CON, 10 INT, 12 WIS, 20 CHA.
Expertise: stealth (+15), perception (+13), investigation (+12), insight (+13).
Spells:
Cantrips -- prestidigitation, vicious mockery, minor illusion, mage hand
1st -- detect magic, identify, comprehend languages, unseen servant, dissonant whispers, silent image
2nd -- suggestion, detect thoughts, alter self, invisibility
3rd -- bestow curse, dispel magic, non-detection
4th -- dimension door, polymorph, improved invisibility
5th -- modify memories, scrying
6th -- true seeing, soul cage
7th -- teleport, plane shift
8th -- mind blank, glibness
9th -- true polymorph, wish
Your sorcerer example cannot be flying all the time, for starters. His range is good but lacks the ability to find the target on an infiltration build like this.
It doesn't matter how many search actions you think you might have. There's no reason to be searching at all. The mechanic is the invisible bard's stealth check (minimum roll 16) against a 10 DC passive perception.
The bard has true seeing illusions are not an issue.
The invisible stealthy bard sneaks in for an initial ambush, spends the surprise round casting dispel magic, it cannot be counterspelled because the sorc cannot take reactions while surprised, every 3rd level spell and lower on the sorc is gone, the bard makes a dispel check for every spell 4th level and above, the DC is 19 or lower depending on spell level, glibness minimum 15 roll +5 CHA bonus +3 JoaT gives a 23 check minimum removing every spell effect from the sorc, the bard's initiative roll is d20 +3 DEX +3 JoaT + d12 peerless vs the sorc's d20 +2 DEX - d12 cutting words for first attack most of the time, the bard true polymorphs the sorc into an inanimate object 9 timed out of 10 and the sorc with no more contingency has the stats of a turnip. The bard can plane shift or teleport away, or rely on a contingency spell using wish from a previous day in preparation.
I gave my bard subtle spell via feat so even losing initiative allows the second spell.
It doesn't need to be those spells either. That's just the example from weak amti-stealth and targetting weak saves.