5x5 deadlifts is generally too much volume and probably constitutes overtraining for you if they're sufficiently heavy. Admittedly, when you're not particularly strong it's less stress and more recoverable (because they can't be that heavy).
Not being able to deadlift 405 after three years is generally indicative of poor programming (if you're trying to get stronger, at least) and/or poor recovery. You have to eat, you have to sleep, and you have to increase the weight each time.
Being 40 isn't really an excuse. You're 40, not 80.
Try looking into 5/3/1, Hepburn style programming, or Tactical Barbell.
Everyone will have their own methods here, but after years of having marginal growth per year I changed it up.
I now only do 1 set per exercise, 3-4 exercises per day. For arm focused exercises I do 15 reps in 1 set. For everything else, I do 20 reps for a set.
If I succeed, then I up the weight for the next exercise (2.5lbs for arms, 5lbs for everything else).
If I fail, I do a drop set and shave off 20% per set taking just 30 seconds per drop.
I use the drop set as both the thing to help the growth, and the thing to punish me for failing to increase the initial set. If I get my 20 reps, I get to move on, so I push really hard because the drop set totally sucks.
If I fail 3 weeks in a row, I drop the weight.
Been doing this for the last two years and I am 47, and each year I have increased my strength by 20-40% depending on the exercise.
I also keep the number of exercises per day to be minimal. With 5 days in the week, I do just 3-4 exercises per day. M, W, R are 1-2 from the 5x5 program (squats, deadlifts, bench, should-to-overhead, rows) along with some extras that I want to focus on. Tues, Thur are more extras, and ones that I am willing to sacrifice if I have a busy week. This means I can still hit the gym just 3 days a week, and hit the core exercises. Also, the core exercises are rough, so Tue/Thur are almost like easy days.
Here is my schedule:
M: Bench, 20degree lying dumbell curl, Squat
Tues: Lat Raises, Hip Thrusts, Neck exercises (put a plate on your head and do lifts from 4 different angles while lying down)
Wed: Overhead press, Deadlift, Cable Row
Thur: Tricep Rope Kettlebell swing Lat Pull down
Fri: Dumbbell Bench Dumbbell Shoulder Barbell Row Front Squat
You'll notice their are some repeats in there with cables and dumbbells. This is helps hit the muscle groups twice a week, but with some variation and some different stabilization. I chart all this in a google spreadsheet with each column being a week so I can see my progress, and so I can see if I missed anything from the core days in the week, and I happily change up days so I ALWAYS hit the core 3 day exercises.
185 lbs is 83 kg; 405 lb is 183 kg.
As someone who has been doing a 5x5 deadlift set every week (plus other stuff) for about three years: this is a wild exaggeration that sounds like some sort of supplement marketing claim. Perhaps you forgot a few hidden variables? For example, I'm over 40.
(It was also very apparent when I had COVID and was out of the gym for over a month that I "lost" about 25kg from my working weight)