Even 100% Eidolons will be quite powercrept in the near future.
Upon initial release, the coming wave of Gacha Eidolons will boost their element by a base of 100% plus an additional 4% for each Eidolon of the same element equipped. The 4% bonus per Eidolon is a fixed value regardless of Limit Breaks, and in this way they work differently than Anubis and Managarmr who boost by 80% plus an additional 4%-8% per Eidolon depending on number of Limit Breaks. This means that this new wave of will have the exact same 100-120% elemental attack increases as Belial, Hraesvelgr, Kirin, and Rudra even without any other Eidolons with a matching element which only provide a further bonus.
To make matters worse for every current Eidolon besides Kaiser Dragoons, these new Eidolons will be buffed to a base of 120% with the same fixed 4% bonus per Eidolon of a matching element. With no Limit Breaks and no Eidolons of a matching element, they increase attack by 120%. At Full Limit Break, they increase attack by 140% on their own. At full Limit Break with a full set of Eidolons of a matching element, they increase your element's attack by 160%.
The first of this new wave of Eidolons is Hanuman, who is scheduled for release on February 7th, exactly a month away. Waiting a month and re-rolling specifically for Hanuman would dramatically increase your power ceiling to unprecedented levels going forward - especially after the buff towards the end of the year when she will be at bare minimum a 120% Eidolon, a 140% Eidolon in the same way that Anubis and Managarmr are 100% Eidolons, and a 160% Eidolon at full power. That's at least 40% more at full power than anything that currently exists, there simply is no comparison.
Because their boost power varies so widely, I don't think it'd be fair to call Eidolons like Hanuman 100%, 120%, or 140% - but rather "2019 Gacha Eidolons". They were designed to powercreep every other existing Eidolon, and they will.
Sanahtlig's guide is unfortunately becoming increasingly obsolete. It was already behind the times in 2018, but mentions nothing of what is to come in 2019. It had some of what was to come in 2018, which makes sense given the perspective from which it was written which seems for the most part to have been 2017 and he does not seem to be here to maintain it.
Will there be an even stronger wave of Eidolons in 2020? Quite possibly, it will be interesting to see what happens as DMM's 2019 unfolds. They will likely face the same powercreep problems on the DMM version, we have the advantage of seeing the future. What this means for the present though, is that it would make much more sense for New or Intermediate players to hold off for a 2019 Gacha Eidolon rather than settle for any of the 2018 Eidolons including Anubis, Belial, Kirin, Hraesvelgr, Managarmr, and Rudra. Veteran players too far invested in their current accounts would do better to save up Draconic Eyes, Magic Jewels, cash, or any combination thereof for when the 2019 Gacha Eidolon of their element is released. I mention Draconic Eyes because if all else fails 150 of them will be able to guarantee whatever Eidolon you want, including the 300% Eidolons that may or may not come out in 2025 if this game makes it that far.