Cool, hey Tommy! Probably around 30 years since we spoke last, you would remember me under a different handle back then. ;) Stinsen (AKA "Mindshadow" back in those days :D)
Would really love to hear some SIDs from you! For the topic on hand I don't have much advice to give, but if you decide to go for GT I gathered a few of my song sources in a pack, could maybe be interesting: Song Pack #1
One good choice would be Cheesecutter CheeseCutter 2.9.0
Regardless of the editor used, SIDDump V1.08 is an excellent companion tool that (accurate enough for most purposes) shows what gets sent to SID.
I wouldn't recommend using CheeseCutter today. It's not being developed anymore, and it uses more CPU time than GT2 and SF2.
As you suggest yourself, I would choose GT2/GTUltra or SF2 if it has to be a cross-platform editor. GT2 is the more popular option and uses less CPU time than SF2. It's also possible to do multispeed as well as tunes for more than one SID chip, neither of which SF2 currently supports. (Multispeed is being developed for SF2 but is not ready yet.) Choosing between GT2 or SF2 is mostly a question of what kind of track system you like. If you want the track numbers to be edited in their own table, with sequences displayed one by one, go with GT2. If you like all sequences stacked on top of each other with track numbers in the left side of each sequence, go with SF2.
Couple of years ago I tested and made music with 12 different music editors.
Once I tried SF2, I felt I'm at home and it immediately clicked with me. This means, to my workflow and brain-capacity SF2 is the closest and best thing. Which, in your case might be different.
I'd suggest you to give GT2, SF2, CC and SID-Wizard a testrun, and not only a testrun but use them for a couple of weeks. Then you'll have a feel for one of them, that's gonna be your tool for C64 music. See, if you settle for one that is not right but people said it's the best... well, then you will feel frustration every time you start making music.
All of the above mentioned editors have great documentation (also, a SF2 tutorial from me is long due and it's also WIP). If you have any question, we surely can find you the best person to help let it be about GT2, SF2 or any of the music tools.