The J2 Core’s stock recovery rejects any modified boot image. TWRP allows you to flash Magisk.zip directly. Root access lets you install Greenify or LSPosed to aggressively manage background processes, making the 1GB RAM surprisingly usable.
If you decide custom recovery isn't for you, restoring stock is simple: twrp samsung j2 core
twrp_j2core_3.4.0-0.tar