This whole system is new, so I could be very wrong in my current thinking.
I think the era levels are not worth investing in at all. If you are a long-time, 13M player, gearing new toons up to G13 should be essentially automatic on passive gear gains in other modes. The only thing era levels get you are gear levels when the era ends and more currency to buy R10 mats. But, if you're a long-time, 13M player, I assume you also don't chase for a ton of 100% maxed toons. So what are you going to do with some extra R10 mats?
For me, a long-time, 14M player who spent some in the past ($600 lifetime but not a penny since DCs were announced), my current plan is to farm the marquee shards/tiers like I always have, auto the coliseum battles to get the rewards I get, hoard R10 mats until they're required or I have a surplus, and let everything else fall in place.