@Astra178 You are aware that Rimuru's transformation into a True Dragon did not change his stats, and only granted him a higher Soul and new Skills, right? (By the way, it is not correct call this evolution. Well, it was more correctly a change of existence, from a True Demon Lord to a True Dragon).
Rimuru's Existence Value would only increase depending on whether with this transformation his quantity and quality of Heart Core information increased, and that would only happen, based on the factor that Raphael would correct certain imperfections and expand the knowledge of Skills and Magic based on Rimuru's existence as a True Dragon. I estimate that Rimuru's Existence Value would increase to a maximum of 5,000,000 EP. (His Existence Value after his Awakening to a True Demon Lord was +4,000,000 EP)
In addition to this "low" increase, Rimuru would acquire the Skills and Intrinsic Factor of a True Dragon. And Raphael would acquire greater knowledge about Rimuru's current Species, and certain information and Skills related to Rimuru's Species as a True Dragon.
Rimuru's power, and his existence as a whole, would be on a significantly higher scale than Rimuru's after his Awakening as a True Demon Lord, and depending on his actions, Rimuru could acquire an Existence Value equivalent to the 10 million EP mark until his battle against Velgrynd and Veldora, before Ciel's birth and his canonical transformation into a True Dragon. (The high EP that Rimuru acquired after his battle against Velgrynd and Veldora, is in short, his +8,000,000 EP that he had reached after the evolution of his subordinates, which granted him more power + Veldora's Existence Value, and + Velgrynd's Existence Value at that time). In other words, Rimuru in this story would only be able to reach an Existence Value if it is the host of Veldora's power, it can thus reach an Existence Value close to 100 million. (But, Rimuru cannot take Veldora's EP, and Veldora remains with some Existence Value).