They still make phones that take SD cards, flagships even. If you decide to buy one that doesn't, that's on you.
They are indeed different projects. GrapheneOS is maximum security possible. DivestOS is best effort security of old/EOL devices to prevent them from being completely e-waste.
> so one is limited to generally the apps
It has the only unprivileged microG implementation, please read this: https://divestos.org/pages/faq#appCompatibility