How to permanently move application to /system partition?

I'm running Android 6.0.1 on Galaxy J5 (SM-J500FN) and the main issue is disk space. If a system app is updated, old .apk remains on /system partition and the updated one is installed into /data partition. You may permanently remove useless system apps and free up space on /system partition, then convert user-installed apps into system apps using Link2SD. But after reboot all settings are lost because these apps are reinstalled again. When you update these apps again, old versions stay in /system and new are installed into /data. You cannot change this behavior, can you? I found another trick:

su
mount -o rw,remount /system
mkdir /system/xapk
mv /data/app/pl.blinkee.mobile-1 /system/xapk
ln -s /system/xapk/pl.blinkee.mobile-1 /system/app/pl.blinkee.mobile-1

You cannot make hard link between different partitions but you can make symbolic link. However, after app update, this link will be overridden and updated .apk will be installed into /data again. Workaround is to set up cron job that moves these apps into /system and makes symbolic links in /data. The best idea seems to repartition drive, shrink /system, extend /data. Any safe way to do it? BTW: What suffix -1 or -2 means at the end of folder name?