Now, before we begin with the list, I cannot stress this enough:
INSTALL TWRP, REPLACE YOUR ROM WITH LINEAGE & MICROG, ROOT IT.
This is essential in order to debloat any Android device. Even if there is no TWRP/Lineage ROM for your device, root it by an accepted method and debloat it with /system/app mover and Lucky Patcher.
If you value your privacy, you will do as advised above. Normal factory provided ROM's from most vendors have so much telemetry and bloat, that it is ultimately faster and more secure to just nuke everything and install an alternative FOSS ROM. Another benefit of that is the expanded support timeline and security patches, with which your phone can last more than the usual 2 year replacement period.
In general, my FOSS philosophy towards phones isn't as strict, since a lot of the development of FOSS applications isn't done for phones and since they are more socially popular than even and financially exploitable, a lot of the apps written for Android are proprietary. As a result, I have limited my usage of the phone appropriately and never access anything valuable from it. I take it, more or less, as a social device - not to be used for computing.
There are a plethora of reasons not to trust Google. Granted, less than Microsoft or Apple, but still a lot. This is why you should aim to rid your phone of Google Services. Along with MicroG, you would be better off installing both of these apps.
F-Droid is a third party repository of apps, that are entirely FOSS. It can update repositories automatically and prompt you to install updates if you say it should. The app also supports adding other repositories if you or your friends happen to have a private one
Aptoide offers a repository of cloned apps from the Play Store with the option of verifying integrity. It offers a person who doesn't even want MicroG, access to most applications on the Play Store.
What's awesome about Discord is that the development on the Android versions don't stay a lot behind the one for the Desktop version. Features are usually ported in a month and users enjoy a relatively good stability. Options like Screen sharing are still in development, however there are previews.
In the case you don't want to deal with usernames and want to be able to send texts to phone numbers as well, try WhatsApp. Now, I would certainly not reccommend its particular use, but hey, it's better than Viber and the others and there are custom patches under LuckyPatcher for it which makes it suck less ass.
File manager: FX File Explorer
With ES File Explorer becoming a bloated mess, including a lot of "functionality" that no one needs and cannot disable, there is a need for a replacement. One really good app that does that is FX File Explorer. It provides a similar interface and functionality, it doesn't require excessive permissions and can browse root. There is a paid version as well that grants additional extras that a user might want to look into.
File backup: TWRP
TWRP can serve for backing up files and the system in general as well as flashing and managing ROM's. It is a very useful tool that copies and archives your information so that it could be retrieved later either by TWRP or by any other compatible unarchiver, such as 7-Zip.
Maintenance: Lucky Patcher
Nowadays, a lot of the apps on Android will have some sort of Ads on them. This in itself, is fine. There are a lot, however, that don't simply have ads, they're infested with them to such a point, that the whole app is literally an ad. On top of that, a lot don't end up clearing up their ad content and cache after it has been served, so it just ends up wasting space on your phone.
LuckyPatcher can be used to circumvent this. It has the ability to patch other apps (if you're root) to remove ads and extend the functionality of others with "Custom Patches". It is a community project, so everyone can review the code and submit patches to get implemented with it. A lot of Android Power Users would advise you against this, since LuckyPatcher is considered "hackware" and "warez" because it features InApp emulation and patches, that can unlawfully grant you a full version of some apps. In the end, it is your decision on how you want to use it.
Video player: MX Player
MX Player has been an old favourite of mine due to several reasons. Even going back as 5 years, MX Player was still the best player around. That is mostly due to its streamlined interface and huge format support, which is why some call it a "Single tool toolbox". It can be used as an audio player, stream player, video player and you can even view pictures with it. It is so versatile that you could argue that if a phone is sold with only it available as an app, people would still buy it. And if that wasn't enough, there is a Pro version of it, that adds even more functionality on top.
Audio player: Poweramp
PowerAmp is one of those hidden gems everyone knows about. If you listen to music on your phone a lot, you probably use it or at least, have heard about it. PowerAmp features a slick interface, combined with a clever use of themes and some nice visualization options, several standard DSP effects, Browsing by folder, as well as other criteria and it even supports external DAC that you can attach to your phone. You can customize pretty much anything you would be able to under a normal audio player on a PC. It too has a Pro version, which is definitely worth getting.