Rather than wonder about a jig, I would wonder about your process. And I mean that in a constructive sense - when I first started doing this kind of work, fret leveling seemed a bit terrifying and overwhelming. I had a hard time being consistent from end to end and side to side when I did my first neck or two. I was just nervous the whole time and the results were sloppy. But after a few necks, it became much more natural. I'm no pro by any stretch, but I am at the point where I can level frets without thinking about it, for the most part. It's one of those things that's hard to explain, but will click once you've gotten it, and then you'll be like, "huh, this isn't a big deal after all." So maybe you just need more practice. Because if your technique isn't there yet, a jig isn't going to solve anything.
I't also hard to speak to your exact experience with that kit bass, because it could have been "difficult" for a number of reasons. You may have leveled the frets just fine, but the neck had other issues. For instance, if a fret is loose in it's slot, it can act like a spring, and it'll compress downwards when you're leveling, since the file pushes it down into the slot. But then, the fret will pop back up afterwards when there's no pressure on it. And when you play the bass, you'll get buzz. Even if you have perfect leveling technique, a fret that acts like this will seem high no matter how much you try to level it. And a neck jig won't solve that problem. The fret needs to be replaced with one having a wider tang, or it's tang needs to be crimped, or it needs to be glued into it's slot. It can be hard to spot a loose fret like this, so you might just assume you need to level it more, and then you'll get frustrated when it seems like you're leveling correctly but still ending up with a problem. If your neck had problems like that, you may have been mis-placing the "blame" on your technique.
Regarding the bass you need the level on now - what issues does it have? Are there are few high/low frets, or is the whole thing worn out? Is the neck structurally intact and true (other than the frets)? If you've only got a few bad spots, you may be able to fix them without leveling the entire thing. Or there may be other problems that need addressing aside from any leveling.
Regarding jigs - I can respect that some people like them and use them for every neck, but personally I don't have a need for one and it strikes me as extra work. All necks change shape under tension - but well designed necks change shape in a nice predictable curve, and you can then adjust the degree of curve with the truss rod and end up with a predictably shaped neck. The key is that there is a known relationship between the relaxed shape and the shape under tension. A jig isn't adding any value on a neck that behaves well like that, since you can level it dead flat when it's relaxed and you'll end up with a perfectly curved shape under tension. Jigs only really help with necks that that are unpredictable - they don't curve in a nice smooth shape under tension for whatever reason. In those cases, the jig lets you work around the lack of predictability by letting you true the frets in their final shape, rather than in a relaxed shape that you know will behave predictably under tension. But even with an unpredictable neck, you can achieve the same end result just by measuring/observing where the wonkyness is, and over- or under-filing certain areas of the neck when leveling.
If you have a priceless or sentimentally important bass that has a really wonky neck, a jig or plek job probably makes sense. Otherwise, I'd rather either fix the root cause of the wonkyness or just replace the neck. And if your neck isn't wonky, you'd probably be better off forgetting about jigs and plek jobs, and just focusing on troubleshooting whatever issues the neck has, and/or refining your level technique. But either way, a jig strikes me as an advanced tool for troubleshooting known problems, not as something a beginner needs to worry about when first learning to do fretwork.
