Anyways some other news just went to Tieba and I just saw a user say that there was apparently an interview that said Ultroid Zero was based off of Zero because he was considered the strongest being to have appeared in Ultraman Z. So seems like we may or may not have outright confirmation that Base Zero > Reiwa Ultra Brothers if this interview really was a thing.
Edit: Found it in Chinese. It's an interview from the 286th issue of the Figure Ou magazine.
tieba.baidu.com
在最初的构成中,最后的特空机是以假泽塔奥特曼为预定进行的。对人类来说是究极兵器,也是模仿这个世界上最强存在的形态。这样的话我觉得模仿泽塔奥特曼的样子比较妥当吧。但是上面来了个‘如果是冒牌角色的话希望以赛罗为基础’的要求”
虽然在这方面做了很多努力,但最终还是决定以赛罗为基础进行设计。仔细想想,因为是赛雷布洛制作的,所以模仿最强的赛罗的样子并没有非常不自然……我说服自己然后开始工作
"In the original composition, the final Special Airborne Armor was supposed to use Ultraman Z as a base. It is the ultimate weapon for humans, and it is also a form that imitates the strongest existence of this world. In that case, I think it's more appropriate to imitate Ultraman Z's appearance. But there is a request from above, "If it is a imitation character, I hope it will be based on Zero"
Although a lot of effort went into this, the decision was finally made to base the design on Zero. Thinking about it, because it was made by Celebro, so it wasn't very unnatural for it to imitate the Zero, the strongest... I convinced myself with that and started working."
So Ultroid Zero is supposed to imitate the strongest existence of the Ultraman Z world which was originally referring to Z. Which I guess makes sense since Ace and Zero aren't actually part of the Ultraman Z world since they only really cameo'd for a bit. It's not really full confirmation but Taguchi is indirectly saying that Celebro based Ultroid Zero on Zero since he is the strongest so in a way its just another indirect statement about Base Zero being stronger than the Ultra Brothers like Ace.