An oem phone battery is good for about 1000 cycles (charge/discharge) before degradation is noticeable. Think of it like age where 1000 cycles is like hitting middle age. You don't move around as well as you did as a teen but you can still do most of the stuff you've always done. You also start to lose your shape and develop a
bulge.
There are ways to prolong the battery, but at 4-5 years, charging daily, you've sailed well past the best before date of battery life. Battery wear also increases exponentially as you end up charging multiple times a day, further driving up the cycle count. So yes, its possible that you feel that the battery got worse almost immediately, but only cause you missed the signs.
As has been mentioned earlier, definitely check to see what apps are using all the power, even when new. Apps misbehave all the time whether it is a software bug, glitch or too aggressive of a sync/update schedule.
Charging your phone all the time via battery bank is not a solution to reducing the cycle count on a new phone. Its a great iron lung to your phone battery on it's death bed. About as inconvenient as one too.
5 year old iphone, with it's battery replaced, could go a bit longer, but there's other issues like screen brightness/pixel burn in (another thing that goes with age), button wear, supported software updates, etc. Repairing consumer electronics is not like repairing a classic Norton Commando. They were made to be disposable and not keepsakes. Your calculation is about cost to fix now and budget to replace.
I replaced the battery of my Note 9 this year for $70. My calculation was that I don't have a need for a faster/better/greater phone than what I have right now and I have better places to put the $2000 that it'd cost to replace with an S22 Ultra. Phone is in mostly good condition owing to having a decent case on it.