https://f-droid.org/en/packages/be.ppareit.swiftp_free/
I guess you can install in on multiple devices (Android phones and tablets), then use a file explorer that supports ftp or use a dedicated ftp client, to then transfer between each other. But accessing my phone from my computers via ftp (Fedora and Windows 10) is a piece of cake and reliable - don't even need anything like filezilla.
To stop changing ip's: I recommend having a static ip range in your router instead of using dhcp (or some routers/dhcp server can allocate the same ip address to the same mac address), so all of your devices ip's stay stable (on newer phones, you have to disable the random mac address feature though).
The ideal situation would be some universal airdrop which will never happen. The next easiest solution is to use cloud storage and send a link to the other person.