Transpilation won't get you passing 99.8% of a comprehensive test suite of a 700K+ codebase in a week (and maybe none at all) and that's assuming transpilation is practical for the pair in question. So if you remotely want these kinds of results, then you most certainly do need an LLM.
There are literally formally verified language transpilers out there today. They can get you 100% coverage without "cheating" like LLMs tend to do by modifying test suites to pass, etc.
I'm currently using an LLM in my day job to accelerate such a 1:1translation, and it's certainly "working"/making progress but God I wish I had a formally verified machine translator instead of this probalistic bullshitting LLM.
Don't get me wrong, it's extremely helpful and impressive in what it can do. But I trust it somewhat less than if I had done it myself, and for good reason. The lies I tell myself tend not to take down production. The lies my LLM tells me do however.
I mean No-one is forcing you to not use a transpiler right? If it was quicker to use one or build a specific, limited one for your existing codebase and run it then you would certainly have done that already.
Sadly none is available for my current use case. Building one is so far out of scope that it'd be the most epic yak shaving of all time. If this was a personal project I would consider it. My personal projects are all about the journey and not the destination so side quests are all part of the fun. Not true for my day job however...