I'm sorry, but there really is no highly convenient tool or method out there to do this. SPC2MML is one of the most accurate transcribers out there for turning an SPC file into something that can be inserted with AddmusicK; and even better it prepares the samples for you and does a good bit of the hard work.
Unfortunately, SPC2MML does not loop correctly, and that's mostly because almost all of the notes are "broken", b=20 or g=31 to name some examples. These notes aren't "broken" parse, but they're not exactly easy to read either.
You don't want to be redirected to Wakana's Music Porting Tutorial, but unfortunately there is no way to actually repair/fix these ports without going through the file and manually changing each of these broken notes to their standard and do looping yourself, since looping is one of the biggest problems when using this tool.
There's a saying I live by: You get back the amount of effort you put into something. Though it took a lot of effort to make this tool from Vitor Vilela's part, using this tool is essentially the "Easy Way", where you can mostly get a port of an SNES game without much cost in time or effort on the user's part. As a cost however, you have to accept that these ports might not be accurate 100% of the time, eat up space and don't loop properly.
I think you can at least help the spc2mml ports a bit by setting the tempo properly before conversion. Iirc, it will try to set tempo automatically, but I'm not sure how accurate it is, so I often use manual tempo. You'll still get really whonky note lengths, but they'll be closer to the intended lengths i.e. c=11 is just a bit shy of c16. In the past I've done a few simple find/replace operations to clean up the odd note lengths at least, and while I still had to deal with horrible desync afterwards, it was a lot easier to spot once the lengths were more reasonable to look at.
That said, I haven't published any of my spc2mml ports. In fact I deleted what I started because I was never motivated enough to finish optimizing, and was never super good at reading mmls which were not my own.
A better solution to the spc2mml problem might be to use vgmtrans, or the myriad of spc tools for various company-specific games (nintspc, rarespc etc.) Not sure how well they work though, I haven't used them.
spc2mml is useful for other things though, so it's far from useless. Echo parameters, adsr, sample extraction etc. It works really well for that.
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