As soon as his spying training finished, he left the Otogakure village and the Land of Sound. He traveled to the spot in the Land of Fire where the secret entrance to the tunnel laid. He was extremely tired from all the running, and his illness was acting up again. He wanted to get some rest as soon as possible.
Many hours later he got into the Konoha. That secret tunnel leads to the inside of some abandoned Konoha building. He was already dressed up and looked like Kabuto. It was a broad day now. He cautiously exited the building and blended into the crowd. He looked for and soon found Kabuto's old apartment which was located under the number of 40.
His teammates were already under Orochimaru's and Kabuto's control so he wasn't worried about someone finding out about his disappearance.
He laid on the bad and again sorted out all the information he had in his brain. He was now 10 years old. Original Kimimaro was 15 years old when the Orochimaru's invasion on the Konoha started. This means that he has 5 more years of Chunin exam failures until the invasion, just like Kabuto in the original.
He also needs to bring Orochimaru an Uchiha vessel. And the only remaining Uchiha in the village is Sasuke. So he will have to somehow manipulate Sasuke into going to Orochimaru sooner than in the original.
He also needs some Hyuuga DNK and Hyuuga Byakugan eyes. Hyuuga Byakugan being only obtainable from the main members of the family, and the only one he knew was Hinata Hyuuga.
He also had to somehow convince Might Guy to teach him the 8-gates. The combined difficulties of all of these things, can't be explained by mere words alone. But he wasn't the one to give up without a fight.
He decided that the first thing he will do in Konoha is to get some Hyuuga DNA for experiments.
It was a fairly easy thing to do. After a couple of days of wandering around the streets of Konoha, he came across a Hyuuga guy sitting around in bar drinking something. He was waiting for him to leave and silently took the glass he was drinking from when he left and brought it to his apartment.
He brought his test equipment from the Orochimaru's base into Konoha. And, after a few days of researching, he managed to duplicate the Otsutsuki genes in his body with the help of the Hyuuga genes.
He now had around 50% of Otsutsuki DNK in his body. Once his body adapted to it, he would get rid of this illness for good. He was now very ecstatic. As this was his first step of changing Kimaro's fate and getting strong in this world.
He realized that his bones became even denser after this. And he started working on some new techniques he already visualized before in his head. He also had a special technique in mind that could kill almost anyone.
The next year was spent like this, doing various low-level missions. Training on his new jutsus. Sending news to Orochimaru. And of course more plotting like always. His next mission is stealing Byakugan after all.