If you really want to keep your current phone you could use a different ROM. One I know off the top of my head is /e/. I don’t believe it’s quite as hardcore about security as Graphene, but it might be your best option (or at least the cheapest one).
As for why Graphene only supports Pixels is supposedly because they’re the only phones that support relocking the bootloader.
Edit: I forgot to mention that /e/ only works on Samsung phones with certain processors, so if your device is from certain regions (such as America) it won’t be compatible.
If you really want to keep your current phone you could use a different ROM. One I know off the top of my head is /e/. I don’t believe it’s quite as hardcore about security as Graphene, but it might be your best option (or at least the cheapest one).
As for why Graphene only supports Pixels is supposedly because they’re the only phones that support relocking the bootloader.
Edit: I forgot to mention that /e/ only works on Samsung phones with certain processors, so if your device is from certain regions (such as America) it won’t be compatible.