Everyone should not worry, I also worried about Tokisen's future after Nagasaka Mako graduated, they also worked with 4 members for a while before adding new members. Now, they are doing well.

But in the Tokisen's case, FOR ME was very necessary the add of new members, Kanamin practically did not sing the songs, in Muteki no Uta she not even sing, just say a phrase at the end, of course, with the exit of Mako, they managed to hold on, but it was clear that the group was not 100%

huynq1995 said:

Due to poor physical condition, Shina Ruka will leave Rock A Japonica and Stardust Promotion

http://www.rockajaponica.com/sp/news/2019040801.html

And Uchiyama Ami

what the....

Do you know when is her last live?

The contract with Stardust will finish too

https://natalie.mu/music/news/329164

OMG Ami is leaving too???? Nooooooooo

On 4/29/2019, ROCK A JAPONICA will be holding a live concert called [ROCK A JAPONICA FRONTIER LIVE ~ Nakano Sun Plaza Saigou Idol Concert~] at Nakano Sun Plaza. It was announced that Uchiyama Ami will be withdrawing from the group and end her contract with the agency.

On 4/23/2019, during the broadcast of FRESH LIVE, it was announced. "There are various things I want to experience apart from idol activities. This feeling has grown stronger every day and the gap just keeps closing. I don't want to lie forcing myself with doing these activities", says Uchiyama citing her reason for her withdrawal. "When I decided, it wasn't because I didn't like being in the group. Everyone's warm cheering and atmosphere, sharing the vibe with others makes me really happy. I love the fans and they're very important to me. Because of that, I've decided to leave it that while I still consider them important", said Uchiyama in tears. Before the broadcast, she had consulted with the staffs and members and quote those messages.

On 4/29/2019, before the launch of their one man live, Uchiyama would like to meet and greet the fans in the lobby."I want to make this this final live the best, full of cheers, smiles and lovely sparkling event with everyone!", said Uchiyama with gusto when asked for her plans. The tickets and play guide will be on sale.

Extracted and translated from https://natalie.mu/music/news/329164
I am personally saddened that both were leaving but Ruka affects me more because she didn't really have a choice due to her poor health.
At least I still have my other favorite, Luna still around.
If you're in Japan, go support their last concert as that formation!

What will happened to their 6th single?

The setlist for the concert. They really went out with a bang.
Last part says:
"From members to Amipon
From Amipon to everyone
Ruttan appearance on stage
End of MC
They will be back on summer"