So as far as periodizationis kinda like grocery shopping. Some people have lists some plan as they shop and most do some of both. I have had great success with planned training and success with Winging it. I will say I’ve made better pure strength gains with plans, but also end up really... really bored in about 6-8 weeks.
When I train with no program it winds up being a lot like what I see of a lot of scripted ones, with several load and deload phases. The amount and rapidity of gains has a lot to do with nutrition and intent. If I approach each session with intent to hit it and build on the numbers from my previous sessions I make gains.
I cannot keep that up forever though and normally take a step back on my own - maintain my numbers rather than try to improve them. If nothing else derails my training I make another run at it after a week or two, or drift off to some other mode or form of conditioning.
That said, I don't use scripted programming, normally get variation by rotating different exercises and/or rep/set patterns on a daily, with intent to build or maintain loading or rep count as I go.
Staying injury free is the best and most reliable way to make gains and keep them.
In the article I linked re periodization by Stronger by Science, it looks as though daily undulating worked the best, followed by linear, followed by none. Experienced lifters got the most benefit.
Greg did a good job crunching numbers on a lot of studies and a couple of things stand out - periodization makes for
quicker gains (over the course of several years there are no studies), no mention of absolute gains. He also found that while benching responded great to periodizing, squat showed almost no statistical difference between undulating, linear, or none. Is a good read.