FP3 - storage too full and phone behaving strangely- how can I scan for virus?

If it gives you some peace of mind, just install the Android version of any of the known brands of virus scanners and let it do a scan. Example (I’m not affiliated, just grabbed this from an internet search): https://www.avg.com/en/signal/remove-phone-virus

Note how even this known antivirus vendor says to first remove any possibly malicious App yourself manually in safe mode before installing the antivirus App to prevent incidents in the future. The weaponry antivirus Apps have while running on the phone themselves is limited, and malware could use this to hide and/or prevent removal.

Consequently a virus scanner should do the scan of the phone storage from the outside, not as an App on the phone, where its weaponry is pretty limited and can be matched by the malware to prevent detection or removal. As a comparison think of booting a Live Linux to scan a Windows installation from the outside.

This would need the virus scanner to use some form of rooting to access all of the phone storage, otherwise only parts of the storage would be accessible (= what you usually get via a USB connection).
Rooted Debugging is available in /e/OS in the Developer options (tap the Build number in the settings a few times to make them visible) at least on the dev release channel (how to check).
But I’m not aware of a ready-made antivirus solution using this.
(Could of course just be a lack of knowledge on my part, since I never needed something like this and didn’t find something like this in the internet quickly apart from this old Python script here.)