Yup, all the ‘more private’ ones like SimpleX or Session don’t have as many features and are have pretty suspicious funding sources (or involved in crypto).

WhatsApp might be private but there’s no way to be 100% sure, since it’s closed source and can leak some messages in reports which is intentional.

Yes and no about Signal supporting multiple Android devices, the Molly fork might be able to if you set it up as a linked device, and we know that tablets can be linked in the actual Signal app.