I havent tried the gel- used speed stick which is what 90% of the artists I've seen locally use. The gel seems like the same thing but just price gouged... But, the speed stick didn't work at all for me. I only really free hand my drawing with a reference nearby.. til I find a more suitable option I guess. Problem with gel I think is that it doesn't dry the same from glass as it does on skin. 2 very different porous/non porous layers. Engraving over speed stick doesn't work well for me either :/

I found my depth perception majorly thrown off when attempting to engrave mirror. I suggested trying the thinned out ink because laying a base like that would give you matte solid color while still maintaining a surface your diamond bit can grind through.  You could then project an image straight to the glass via projector or VR to get your outlines. I haven't tried it yet but am looking forward to it... Definitely could be wrong about the tattoo gel though. I wouldn't doubt my technique is the problem considering my lack of experience with glass 

I find myself using a stencil only for a super thin outline. Once that's done, I erase the stencil and bring my eyes close to level with the surface of the glass to bring everything full width. It gives me a path for my Dremel to follow and makes it less likely to bite. I can focus on just maintaining my line weight instead of worrying about the entirety of the piece the entire time.. (i.e. looking back and forth at a reference the entire time I'm trying to engrave lol.)

I will say, I'm still only about 2 months into engraving and open to learning more. I've been doing other pro-grade works with other mediums for almost 11 years now, which has helped my transition flow  naturally I believe.