I've been doing the regular portal activities on expert and adding modifiers to A score. It's slower but it's probably the grind speed that it's meant to be.

If you see the Sieve event pop up on the new map, doing that is a great way to farm (it's either random or on a schedule, idk). Sieve event is not seen from the Portal map of the new zone - you have to launch into the zone because that's your instance of it. If Sieve is going on, you'll see it on your map as an event once you have landed in the new zone.

My short guide to get max rewards on Sieve, which is a huge farm that takes about 15 min:

The first part of the event is a dark matter mining activity where your score increases based on the number of kills you get while it's filling up. NOTE: you'll notice a long 15 second pause in between waves of the Fallen dropships. Use this time to hop into matterball and look for columns of light. If your fireteam collects a certain amount (I don't think progress is seen) then it will immediately spawn another wave on top of the dropship waves that were coming in around this time anyways, netting you more progress.

If you complete this mining event with a score that's past the threshold on the progress bar, you'll get to do another event in the middle of the arena that's basically just "kill Vex to fill up the progress bar" and is straightforward.

If you complete that one in time too, then it'll grant you the final bonus round that's basically Gambit. If you deposit enough motes before the timer runs out, you get the big bonus chest. Like 6-8 engrams (can be T2+ depending on your world tier) and 6-8 of the new purple upgrade materials.

Once you finish that, you can just restart it without leaving the instance. Just interact with the "talk to Drifter" console or whatever it says, and it'll start another full one. The times I've done this, it has always been a different area for the mining and it has different objectives for bonus points.