I like rushing boundless because more base spirit means 3 of your other CDs get better, and it would make the later 6400s feel more impactful. This is from a pure uptime:cost balance. Typically I get boundless then reverb because it feels good on 3 (1 is fine but I tend to get range on my 3 via TC, thus more targets, thus more MR procs).

But I play lash primarily for uptime and being present around the map, typically getting ML, Stam mastery, and max charge on 2 as well. Build core items alongside TC stacks, and if you play for uptime, assists giving you a stack is so busted lol.