Sometimes I think about playing another MMO, but then I think

[Evergrey]

just try them out man. I also play guild wars 2. Your own character talks with voice acting. That alone makes its story more enjoyable than this game for me. And That game has no sub And no vertical progression. Thus no gear grind. Perfect secondary mmo for me.

I also tend to jump between GW2 and also ESO, Both have no sub (ESO has an optional sub if you'd like tho) so they are perfect like you said as secondary MMOs.
Easy to put to the side for some time and can log back in at any time.
And both have horizontal progression so even if I don't play for a long time I can still jump in and comfortably play newer content.

ESO also lets you glamour/transmog any armor type on any character, no class restriction.
Something I always think of when I see people asking for the same system in FFXIV.