Personally, I find the fact that I remember some/a lot of the cases helpful! There’s still a lot of kanji/grammar in Conan that I’m not necessarily familiar with, so knowing the broad strokes of what’s going on means I don’t have to stress out if I don’t 100% understand a sentence or explanation.
But if you’d get bored from re-reading/watching those, it’s not like you have to go through every single case again. You could always skip cases that you remember really well and stick to ones that you aren’t as familiar with.
And yeah, long series can be a bit tricky to catch up on, lol. But you don’t have to do it all at once, or try to keep pace with new releases. I pick up new Conan volumes when I see them at Lawson, but in my Japanese practice I’m on like…volume 7 
. Sure, I’ve read a lot further in the English, but if I wanna go through it in Japanese it’s gonna be a loooooooong ride.
It’s always up to you, though. There’s so many different stories out there, you don’t have to force yourself to use Conan if the length is too much for you. Like folks have been saying for OP’s question: best to find the immersion that interests you, so that you have an easier time sticking with it and you can learn interest-specific vocabulary.
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