Going back to LP

Hey,

27, Male, 6 foot, 174lbs.
Squat 165kg, Bench 110kg, Deadlift 190kg

Been training using various intermediate programs for the past 10 years, custom U/L, 5/3/1, Candito 6 week, Custom PPL. Strength has been up and down with 10-20kg on all lifts over the past 5 years.

Recently started The Bridge, attempted W3D1 today and squat was down 7.5kg from last week, no change in sleep nutrition etc.

Do I need to attempt a more advanced program with a calories surplus due to the amount of time I’ve been training or would going back to basics and completing SSLP with a large weight gain be the smart move?

Thanks for any advice

You don’t need to change programming or anything else- you need to keep training and gain weight slowly. You aren’t a novice either, so that’s not going to work.