I agree with this person's suggestion but want to put another spin on it. The goal doesn't have to be giving you a brand new voice, just use it to exercise. At the very least, the muscles will be "ready" to speak when you have to so you'll have more confidence about it.

Do you like music? Try singing or even just humming. Play around with your mouth, throat, and posture to hear how it changes the timbre. I won't over promise here but people have WAY more control than they realize. Say "aaahhh" and move your head forward and back, it's crazy what a big change it makes.

I don't think the character voices you gave are fair, though. Filthy Frank and Solid Snake both sound like they've smoked two packs a day since birth. Mimicking that damage is kind of damaging in itself. A similar thing happens with singers who only use falsetto - that is ROUGH on your body.

Point taken about Kristen Schaal. She does have a cute voice that works well with cartoons. But I would absolutely take her seriously in a serious role, too! She's just really funny so she mostly does comedy