Well if you lose power in an unlucky situation where the system is writing down something, no SD card can protect you. But you applications are not that critical to loose real data. They simply can be restored in a couple of minutes. Best is to use dietpi-backup function. Just plug an USB stick into you device and use it as backup taget. This will create a snapshot of you system and you can restore your whole environment if needed. As well a good thing before doing any software updates 
Regarding the r/o fs protection. Theoretically you could use dietpi-drive_manager to set r/o for specific partitions. however I would not recommend it. There are some software title who need r/w access, otherwise they will not start. A better option is to use an overlay fs for root partition. Just have a look to this post https://dietpi.com/forum/t/samba-fails-on-ro-filesystem/4123/2