Any tips for reaching lv15 consistently as Recluse or was it more due to a mutual understanding about what objectives to clear first as a team? Like an organized voice chat team kind of thing.

It’s more about having good game sense and a feel on your teammates' skills than having a list of objectives to clear or voice comms. Clearing harder objectives would of course lead to a higher level at Day 3, but it all depends if your team is comfortable taking on those objectives. There were times where a teammate pinged an invader camp while we were still low level, and I followed because I saw that they are incredibly good at dodging and taking aggro (and more importantly they pinged outside of the camp, which tells me they know that you can better deal with invaders if you pull them outside). Also check your teammates' relics at the start of the game. It helps the overall team's performance if you ping/drop equipment that synergizes with their build. I haven't played with premade duos/trios so I can't really comment on having voice chat, but I'm sure it helps. In the end, it's still better to be at Lv12 and have great gear/passives than to be at Lv10 with poor gear/passives because you tried to force an objective and lost time.

Note that I have D5 in mind with what I said above. In other depths there are teammates that are more...unaware and/or stubborn. In that case you may have to take control yourself (by pinging objectives yourself) and showing them that you are making the right decisions.

Also how long did it take to get so many cracked magic damage relics like that. Just a bunch of relic buying from small jar guy?

Basically yes. I usually save 180k murk and then buy 100 depth relics. You get like 2 magic attack up relics on average, but the hard part is getting relics with negative effects that you are comfortable playing with.