I know I'm really late for this. In my opinion I really think this was probably decided a while back, but maybe due to the pandemic it had to stay under wraps until everything calmed down.
I remember when Kobushi Factory disbanded they mentioned that there were talks about adding more members to the group, yet the girls chose not to so that it wouldn't ruin the group's spirit or essence. Perhaps around that time they must have spoken about this with Tsubaki Factory as well.
In my opinion, I would have preferred a one generation group just like C-ute and Berryz. I personally am tired of all these never ending groups which at some one point forget what the true essence of each group is. I will always prefer originals.
As for whether this was due to Risamaru's scandal or not I really think this was probably discussed before any of this happened or at least thought of. I don't think any of that influenced in the agency's decision. I also don't think things would be 10 times worse had she stayed in the group. Things don't escalate this much from one day to the other. I'm sure there was already tension in the group since before, but the girls are professionals. They did not show this. That's the way it would've continued if Risamaru would've remained in the group. Nothing can assure us there's no tension between some of the girls already, basing it on Kiki's statement that at times she feels that nobody likes her. Of course, this is just her opinion and perhaps that's not the case, but this can also be a sign that there is tension between some of the girls yet they chose to not make it public because that is part of being a professional.
These are just my opinions.
I am not sure how my comment about Risa's popularity led to a discussion about the implications of her return to the group... That is water under the bridge now and it was clearly a point of no return, I wasn't talking about such what-ifs at all, what I implied is that the decision to add new members to Tsubaki Factory is the after-effect of that whole fiasco. That situation caused them to lose fans and the momentum they had up until that point.
No one said the current members are not good enough, they all have their individual strengths. But if the group had stayed as they were and things hadn't gone south, they wouldn't be adding new members, at least not right now. But I think it's a good sign that the office trusts the Tsubaki brand and wants to keep it going for many years to come. It will come at a cost of the group becoming generational, but it worked well with Juice=Juice, and I think it will go over similarly with Tsubaki too.