I've had many kinds of Android phones over the last few years. I, too, have an S8 - great phone. I use Nova Prime (paid for version of Nova) as a custom launcher with it's companion app Teslaunread to show notification badges. My impression is that Nova and Teslaunread together are better than Evie (more customisable) - although I've not used Evie much.
I don't use an antivirus app and never have on recent Android phones. If you only download apps from the Google Play store the risk of getting a virus or malware is low. Although it depends on what websites you visit on the phones browser
Antivirus/Malware software will slow down the phone. If you want to use one, Malwarebytes is good but do a search for 'best Android antivirus/malware apps' and read reviews. Be careful though, because some of these types of apps get deep into the handset software and can be difficult to get rid of. I would NEVER put McAfee on my handset for this reason alone.
Most battery saving apps are not that good and tend to use battery power themselves. I use Greenify to hibernate apps that are persistent battery hogs. With Greenify I also use the 'Aggressive Doze' setting. My phone is not rooted, so setting up aggressive doze in Greenify requires some 'ADB' commands to be run from a PC with the phone connected. I suggest you search for Greenify ADB commands to see if this is for you.
Alternatives I have used are Naptime X, ForceDoze, Hibernate, and Doze - these are all apps from the Google Play App Store. Again some ADB commands may be needed to get the bets benefit from these apps. Mostly these apps force the handset to go to sleep (doze) as soon as the screen is off and can be used to supplement the inbuilt Android doze function.
I also use the 'Mid' Power Saving setting on the S8's own Device Maintenance/Battery settings (on at all times). I get about 2+ days battery time from my S8 - although I have changed some other settings on my phone to try to maximise battery life.
Any further questions let me know.