Good news, I've managed to get my old Caustic 3.2 code to compile and run for 64bit Android, but you'll need to jump through a few hoops.
First and foremost: understand that this is NOT an update. It's just whatever was running 8 years ago, made to run on modern Android devices.

Now, you'll need to be comfortable side-loading an APK, I'm not going into how to do that. Look up "installing from unknown sources" or something similar for your model of phone or tablet.

Next, to bypass all the file handling mess that was keeping me from upgrading Caustic3 in the first place, I've decided to go the nuclear option and request access to ALL YOUR FILES, like a file manager app would.

This way, Caustic can still access the root/caustic directory like it did before (it should even be able to load up your old files and presets if you haven't deleted them).
Normally you can't just do this as Google won't publish your app on the Play Store with this permission, but hey, we're not on the play store.

After running for the first time you should see a system popup asking you to flick the switch to allow file access.
If you don't, it'll just keep asking you so either trust me, or don't install this.

In testing, some users have reported crashes when enabling MIDI.
That's unfortunate, I think something changed in the way USB devices are accessed now, so if it doesn't work, there's not much I can do about it.

No need for the unlocker APK, this one's on me.

Android APK


And if anyone needs the Windows version, here's the "Standalone" version.
There's no installer so it can be run directly from a USB stick or copied into a local folder. You'll find the EXE in the app/bin folder

Windows Standalone ZIP

Enjoy!
-Rej, aka SingleCellSoftware