How do I get to the recommended 623 ilvl doing delves?
615ish you should be fine doing T10s. I’d even be so bold as to say 610.
2 runed crests at at time I guess
Getting to the ‘recommended’ ilvl isn’t really possible just doing delves, at least not in the short term; you have to get really lucky with map drops since those grant 3 gilded crests each at T8+ (maybe T7? Never tried).
You really can’t. Delve gear (other than the vault and the gear from maps) is only Champion, so you’re not getting Hero or Myth Gear outside the Vault. Have to farm Mythic+ for at least Hero’s. That’d the max it gives outside the vault. Championg gear maxes at 619, not sure about Hero gear. Myth gears comes from Mythic raiding or the vault.
That said, the ilevel recommendations are extremely exaggerated, especially if your Brann is a decent level.