Battery Life Question

A lot of those points you brought up are commonly recommended things to do to make your battery's overall longevity better but keep in mind they apply to lab conditions in test research facilities. In the real world, most of us don't have to time or just aren't willing to monitor our phones the entire time they're being charged. So it's a matter of degree how much time and effort you want to spend each day charging your phone. There is of course no shortage of 'battery do and don't' articles floating around online -- some are based on out-of-date info, some just opinions posted as fact, some are relevant and data-based. I'll suggest you go to the Battery University site:
https://batteryuniversity.com/learn/
Unfortunately all smartphone batteries will fail at some point. Most just lose their ability to hold a charge (and will take longer to charge up) but some like yours also swell up. Hopefully there's no physical damage to your S9. After a couple of years it's not uncommon to at least start seeing a battery lose some capacity, but most will last longer. You should avoid letting it drain down to the point your phone shuts itself off. Not an issue if this does occur every so often but don't to it intentionally on a repeated basis as that will drastically shorten a battery's life. Every smartphone battery only has a determined number of charge cycles so the goal is to make that number last longer, not shorter. Leaving your phone on your charger isn't the best way to leave it all the time. Once the battery is charged it will stop receiving power from your charger but leaving it on the charger plate creates a situation where your battery is going to a repeated charge - no charge cycle. Once the battery drains down a few percent it will start charging again. Again, there's the issue of the set number of charge cycles. It's also cumulative so when the battery is down to say 50% if you charge it to 100% that involves one half of a charge cycle. If you charge it only up to 80%, that would only be less than a third of a charge cycle. Plus there's a number of other variables involved (i.e. a faster charger involving USB will involve more generated heat than a slower charging wireless plate). Anyway, leaving your phone on the charging plate isn't the worst thing to do but it's not the best thing to do. Read through some of the articles at the BU site like this one:
https://batteryuniversity.com/learn/article/how_to_prolong_lithium_based_batteries

You might also want to install this AccuBattery app. Note that this app isn't claiming to make your battery last longer by implementing some kind of 'fix' to the existing Android power management (avoid those kinds of apps, they're basically just snake oil), it's just an enhanced information utility. The Settings >> Battery in your phone is good for the basic stats it provides but AccuBattery is much more comprehensive and extensive. It also includes a simple alarm -- you set the level (i.e. 80%, and it's just an alarm, it doesn't halt the charging process so it's still up to you to remove your phone from the charging plate). So it doesn't interfere/manipulate how your battery gets charged, it's just providing you with more data so you can determine how you want to charge it.
https://play.google.com/store/apps/details?id=com.digibites.accubattery&hl=en_US

 

Leaving it on charge all the time is not a good idea. Let the phone run on the battery (which it is anyway even when on charge), charge it when needed and unplug it or take if off the pad.
At home I charge my S9 almost exclusively on the Samsung wireless fast-charge pads which have a built-in fan to reduce heat while charging. This is important as it's the heat that kills the battery over time. WIreless charging can actually produce more heat than cable charging because it is less efficient, the phone and charging pad are essentially the two halves of a transformer and the power transfer is nowhere near 100% so some is lost as heat. Even standard-rate wireless charging can significantly heat up the phone and pad so it's important to monitor this while charging and take the phone off the pad if it gets too warm.