Should Rakan always build tank support even when his real value also comes from re-peel and second phase?

In most games, yes, because his real value comes from playable engages and prolonged utility, not disposable all-ins.

Glorious or Locket on Rakan depending on whether he mainly lacks access or durability after entry?

Glorious if you mainly need to trigger the opening; Locket if you mainly need to protect your team during or right after contact.

When does Force of Nature truly become priority on Rakan?

When AP burst prevents you from reaching a second rotation after engaging. If you connect but disappear instantly, you often need earlier MR.

Rakan Wild Rift Best Build Guide (Patch 7.2a)

Rakan needs value before, during and after entry. A build that is too aggressive turns him into a disposable initiator; a build that is too passive removes the threat that makes enemies respect his angles. The best compromise is giving him enough tools to engage cleanly, enough tankiness to avoid…