@ctolzane so I bought this router 2 days ago. I've found a way to still obtain root access in new firmware:
reading https://th0mas.nl/2020/03/26/getting-root-on-a-zyxel-vmg8825-t50-router/#fnref:1 I understood that all password, even for root and supervisor users are contained into the zcfg_config.json file. Actually in my router accessing this file is much simple: just set up FTP by webgui, access FTP as admin user (I used MiXplorer on Android for example) and zcfg_config.json is just here. The absurd thing: it has rw permission even with admin user! So just open it and copy the encrypted password under the root user (not the default password as per the guide linked, that's the same that you can calculate with the emulator). Now we can decrypt this encrypted password just by using the DynamicDNS as oracle explained in the guide: set a fake DDNS in webgui. Download the backup file. Open the backup file and replace the encrypted password under DynamicDNS with our encrypted password of root user copied before. Now save and restore the file from webgui. Go to ddns settings and just read the password: it's clear. As simple as that!
I still hope someone can build openwrt for it, or if and when I've time I could learn and do it myself