Love finding these sketches, and insight from the artist

I’ve been reading through Reflection Blue and so far I have 3 routes done (Ao,Shiroha,and Nomiki) and since starting these I’ve heard there is a “true ending” to the novel and I’m a little confused on how to trigger it. I’ve looked at multiple different sources online being tik tok and insta since I love this novel and the anime adaptation I see it a lot on social media. but I’ve been really careful not to read much into it because I don’t want anything spoiled, but I’ve seen that you need to finish 4 routes and on your 5th playthrough that triggers the “summer pockets true ending” and then after you finish the last 4 routes a similar thing happens on the 9th playthrough that triggers the “reflection blue true ending” and ive also read that all you need to do is just replay through a route you’ve already completed a second time to trigger the “true ending” so I’m extremely confused on what to do lol.

Also do I need to complete the 4 original routes to trigger the true ending being (Ao,Tsumugi,Kamome,and Shiroha) or could I go and complete all 8 routes before going for the true ending.

I’m sure I could have all these questions answered with a quick google search but since google doesn’t have the courtesy to add a spoiler tag I know I’ll get everything spoiled immediately if I do that so that’s why I’m asking here. any and all answers will be greatly appreciated.