Which legendary should I make next for convenience?

Runes sound like the most straightforward choice here. Since you already have legendary armor, being able to change the armor’s runes can make trying different setups easier. I’d pick them if you often adjust builds outside combat.

Sigils are useful across characters, but think about how you swap weapons. If you change weapon sets during a fight, having to reapply a legendary sigil may be a hassle. Regular sigils on separate ascended weapons could suit that habit better.

A legendary relic could be handy if you like switching relics across your builds, but it costs more. If that flexibility matters less to you than the price, I’d leave it for later and choose between runes and sigils based on how you actually change gear.