It is very unfortunate that so much of the Android ecosystem relies on a very proprietary and cloud-connected piece of software. Without it, most apps simply refuse to work. MicroG has done amazing things here, but unfortunately a lot of things still break when using it, to no fault of the MicroG developers.
Modern bank apps require not just Google Play Services, they also require the phone to pass Safety Net. Play Services substitutes like MicroG aren’t capable of that.
The truth is that service and app developers only care about one thing: audience size, which translates to actual money. And it takes a long time to build that audience, even more so in the mobile world where all metrics are orders of magnitude bigger than on the desktop. Until Linux phones can show they have sizeable audiences, developers will ignore them. Tens of thousands is nothing, we’ll need millions. Realistically, this is not going to change in 12 months or even 2 years.
But, something can be done in the meantime: making all popular app-development frameworks run on mobile Linux, and then putting pressure on developers to produce app builds. When portability comes essentially for free, it becomes easy to support an additional platform just for the brownie points.
download the apk from https://www.whatsapp.com/android/, start anbox (this takes a while), install android-tools, adb install WhatsApp.apk, done. It also works with stuff like Duo Mobile, which is the 2FA my organization uses (which is interesting because it doesn't agree with my rooted lineageOS phone).
Anbox comes by default on current versions of Manjaro