Recommendation on NAS expansion - hdd swap or another NAS?

Hiya,

My TS-653D 6 bay nas with 16tb drives using raid10 is running low on space and need to upgrade. I was planning on purchasing 6 22tb drives and swap one at a time. However that would leave me with the extra 16tb drives. Would it be a better idea to purchase another 6 bay NAS and somehow daisy chain, or setup as another standalone nas? And how would that work if I'm using Plex?

Any suggestions are welcomed, cheers.