If I were to guess why people feel there's a content lull or that there's nothing to do is actually somewhat the fault of the devs adding more content than was necessary.
Tying the relic weapons to Eureka is what, in my opinion, is causing a portion of the playerbase to lose interest in the game. I'm not bashing it or anything, I like it well enough and I think giving it actually meaningful rewards was a good way to incentivise people to do the content, but as much as we complain about the ARR and HW relics being grindy, recycled content and taking too long, the devs kind of shot themselves in the foot by tying the SB relics to new content.

The ARR and HW relics could be completed by simply playing the game, pretty much anything you did would further your progress on them if only by a tiny bit, which meant that pretty much all kinds of players were able to complete the relics no matter what type of content they enjoyed and how they liked to approach it.
It essentially provided you with a reward for simply playing the game more, you could be doing whatever content you felt like and it would contribute towards a certain goal, that being the relic weapon. So whenever you thought "I don't really have anything in particular for me to do today" you could always just work on your relic.

Now in SB the relic is tied to Eureka, and Eureka is content that not everyone likes. It's also content you have to go out of your way to do. The SB relic is completely seperate from all the other content in the game, whenever you're doing other things you're not contributing towards your progress on the relic, and whenever you're progressing on the relic inside Eureka you're not really doing anything to help you outside aside from earning tomestones.
You can't get your relic by simply playing the game anymore, or rather, you can't get it by playing the game how you want to play. This means that people not interested in Eureka simply won't bother with the relic and people who are indifferent to it will just get whatever relics they're interested in and never go back.
So now the answer to "What should I do when I run out of content?" goes from "Just play the game and do whatever you want and you can work towards this goal" and becomes either "Go do this content you might not be interested in and has almost nothing to do with the rest of the game" or "Just play the game and get nothing for it", which much less people will find appealing.

It's not that there's less to do in SB in terms of actually meaningful content, it's just that they gimped what you used to be able to accomplish by playing the game long-term, so now there's less incentive to keep playing.