iPhone SE only lasts 3-4 hours after replacing a battery

Guys, this is driving me nuts. I only get 3-4 hours of web browsing / relatively light use out of my iPhone SE, despite having done the following:

• turned off background refresh completely
• display on auto-brightness, turned way down most of the time
• no push on e-mails, fetch only in 15min intervals
• location services dialled back significantly
• implemented pretty much all the other generic tips you can find on the internet

I've reset all my settings (once), and I've done a full factory reset followed by a back-up (once).

No use. My original iPhone battery was still at 90% (after 11 months of us) based on the health indicator, and supporting peak performance. I also talked to Apple support, trying to have them replace my battery, with no use (only getting the broken record: "your phone is fine, sir, based on our diagnostics", over and over again, never mind that my battery went down from 40% to 25% during the 15min sorry call that I had to endure :-)))

At that point, I decided to replace the battery myself for a new one (non-OEM). I've benchmarked the replacement battery using the Battery Testing app from Appstore, scoring a top performance (as you would expect, with a brand new battery).

Yet even after the battery replacement, when I looked at battery usage, over the last 10 days, I invariably get only 3-4 hours of phone use for 100% equivalent of battery use. This is driving me nuts. Also, I lose as much as 10% of the battery overnight, when the phone is in the airplane mode, hence not doing anything in the background. Only three theories remaining:

• It's a general IOS 13 issue (I don't quite recall anymore if things were as bad with IOS 12, but I suspect it wasn't much better, so I am not inclined to blame it on IOS 13)
• There's some other hardware component in my iPhone which is draining the battery (which would explain why the drain is not showing up as app use in the battery settings)
• There's a stealth app which is draining battery without this registering as app use in the battery settings (but I don't know if this is even possible, software-wise).

Any smart ideas that you could share based on your own experience with a disgruntled iPhone user?