I feel like I gave this a good chance, 25 chapters before I got bored.
The premise is average, but I don't often like to judge solely based on that because an old idea can always be done in a new and fun way... That is not the case here. It feels kinda, lazy? He tames thousands of slimes but never does anything with them but use them to scout places? It seems a waste.
What hurts me the most is the plot though. Not the strongest point for most isekai, but this one absolutely takes the cake. Every few chapters he visits a town, encounters a world-ending problem, solves the problem, gets famous, and moves to another town. I genuinely lost count at how many times he did this, had to have been at least 6-7 times before I got fed up—and remember, that's 6-7 entirely different towns within 25 chapters. If the problems were interesting I might've been alright with that, but if it's a problem that can be solved by him blowing it up then it's not an interesting problem (literally every single one of them was solved by him destroying something btw).
The only thing I really liked about this manga was at the beginning when we were learning all his new spells. Him testing out how strong the sage's abilities were was fun and then he transitioned into being able to create new, original spells—so the depth of creativity should've been endless... But for the last few chapters he's only used the same ones over and over again.
Overall, 2/10—There's no depth of any kind to be found anywhere in this manga, which is a shame because there's a LOT of opportunity for it.