I found at least Buypass offering a gratis ACME product "Buypass Go SSL". They have roots which are deployed at least since Android 4.1, which covers way more Android devices (according to the Android Studio statistics, >99%):

https://android.googlesource.com/platform/libcore/+/android-...

I'm not sure whether that particular root is being used for their Go SSL product. If so, Buypass might be a good alternative to migrate to.

From what I read though, they do require an E-Mail address, so you've got to keep that in mind.