this is like, the bad habit of 99% of new writers. its my bad habit too :u
its also the priority to fix, and fixing it means making something that has a forseeable and nearby finish.
what makes you think your original idea will take too long? is it a long story, or simply a long process? you can change the style slightly to simplify the process if you need to, lots of comics reserve their very fancily coloured pages for special occasions like openings and finales.
if its a long story... id suggest considering putting it on hold to do a shorter story. significantly shorter. like 30 pages and yer done. short stories are way easier to plan bc the end is in sight - often, esp when starting out, long stories can feel like youre drowning in them. theyre so big. a short story is something you can get done, and then have the entire writing process under your belt to evaluate and improve upon on your next project. starting short means youre more likely to finish it, and finishing is the first hurdle - from there you can ease your way up to your epic.
it could be that in that time you grow out of the long story you want to do right now - but thats not a bad thing. ppl age out of their ideas, and find ones better suited to who they are at the moment. since youve started a few pages of this thing though, you could always wrap it up into a pilot - satisfying on its own, open to extension when youre ready. that could even be your short story practice lol.