This is absolutely the WORST advice I have ever heard. Why would anyone in their right mind not tell a kid they are not their real father? Kids getting that bomb dropped on them later in life is much harder than 5 years old. There’s a Peal Jam song you should listen to by the way.
Knowing what you do here’s what I would do:
Get a babysitter and go all out romantic date. Dinner, wine, flowers, maybe a little get away. Go all in an make love to her like your wedding day. Leave her begging for more.
The next day have her meet you where you and your lawyer serve her with divorce papers. Then never look back.