Paying Off Your Mortgage

Hello, newbie here... I've been overpaying my £800/mo mortgage at £2250/mo for a while and i want to bump it up to £2900/mo. My goal is to finish the mortgage in around 5 years. There's £157k remaining at 2.3%

However, i went through the flowchart here and now i think i should drop my mortgage payment to minimum and invest the max £1666/mo in a vanguard s&s isa instead and the remainder in an isa in my wife's name on the logic that it should earn higher interest than the mortgage loan is charged at.

Am i barking up the wrong tree here? Am i off base when i assume around a 3% return on the s&s ISA after fees?

Apologies for a dumb question, i've spent most of my adult life ignoring my finances (to my detriment) so this is progress for me!