You are doing absolutely nothing wrong with that method because sims are designed to take those signals.
The problem is that those Inst ins aren't all created equal AND they are meant to take a plethora of instrument inputs as opposed to just guitars.
The ideal input for a guitar is 1 megaohm which is what tlw was alluding to earlier. That's what generally appears on guitar amp inputs but not necessarily on interface inputs or other devices. That's why things get a little hairy for tone chaser like me. For general input you really don't have to worry about it.
The other problem for me is I want/need the FULLEST and richest signal going into the interface so when it hits the sim I can get it to truly respond EXACTLY how I need. Again not something most guitarists need to worry about if they are just doing a little this or that but all my stuff pretty much revolves around the guitar. I need the absolutely most useable and flexible input signal possible so if I try to dial in a super clangy/brittle/tubey type tone from a sim it doesn't fall flat. Or if I need a crazy thick and responsive hi gain lead sound it doesn't get all mosquitoey or not sustain properly.
Essentially short of dropping thousands on gear (or tens of thousands on gear) just getting that input signal humming a little better will make the sims work a little better which is all I need really.
And yeah... you read that right. What I'm currently doing is a massive PITA which is why I'd like a simple device to input straight into the interface or into the mixer and bypass the hassle of screwing with those amps.
But that is only part of the equation because I do actually need something for microphone and bass input which both have been far more problematic than my guitar stuff. Just that little extra should make mixing a whole heckuva a lot easier.
So really... knowing the type of stuff you have said you are doing I would not worry about these things for even a second. In a few years it may be something you want to look at if you really start getting into live input of guit/bass/vox but just tossing down some tracks to use on your synth based creations? No freaking worries. I'm trying to emulate live recorded band stuff in a one man, low budget, all DI/digital studio. That's why I need to get extra creative about stuff and seek out crap like this.
Don't get me wrong. I've been getting some REALLY nice results as it is but it is a LOT more work than I think it needs to be and I STILL think it could be better.
Tone Chasing. Not Even Once.
lol