I have used the Sigma 18-35mm with a Nikon speedbooster as well as the PL 12-60mm and the Voigtlander 25mm 0.95. My thoughts below
The 12-60mm makes shooting quite easy with its AF and Dual IBIS but it does often render a clinical video sharpness look that cinephiles can't stand. Focus by wire is particularly poor on this lens compared to others I've used (from Olympus' cheap 50-150mm or the PL 15mm 1.7). This is my most useful lens due to its great focal range and it goes with me everywhere. I'm not in love with the look it produces but don't dislike it either - pleasant bokeh when you get close to your subjects. I usually shoot at f4 to soften the look. I'm currently looking into alternatives like the Olympus 12-100mm to get away from the focus by wire. The PL is sharp, renders lots of detail, and very flat contrast wise IMO.
When I use the PL 12-60 interchangeably with my 25mm Voigtlander the difference in lens looks are so pronounced that its as if different cameras shot them. The voigtlander is soft, dreamy, and very contrasty. Manual focus on the voigtlander takes some getting used to but is very smooth overall. Close focus on this lens (and I believe the 17.5mm) is ridiculously good, so the macro capabilities are worth noting. The lens is pretty light (not much heavier than the 12-60) and balances well on the GH5. I much prefer the look of this lens to anything else I've used on MFT, including the PL 15mm,, Olympus 17mm, Lumix 20mm, etc, even if it is soft.
The Sigma 18-35mm 1.8 with a speedbooster could see in the dark almost as well as the Voigtlander, and produces noticeably sharper results wide open. I found it gave a more accurate and less filmic rendering. With the Nikon speedbooster you get a nice manual aperture control ring, which couples with the nice zoom and focus rings that beat out any focus by wire alternative. IBIS gave me issues if not set to the proper focal length, so when zooming you have to manually change this which is very unintuitive. If this works in the Canon mount I'd consider picking another copy up. (Canon metabones users, please share insight into this?)
Mostly, I hated using this lens because with the speedbooster it was 2.2 lbs. The size and weight advantage of Micro 4/3 disappears. This made my camera feel ridiculously front heavy and I'd leave this lens at home a lot. It felt more balanced with the XLR adapter mounted and a shotgun, but still, the size and weight made it really unattractive to use for everyday shooting. I sold this combo.
A couple things to consider that haven't been mentioned so far - consider the Extended Teleconverter (ETC) mode on the GH5 when looking at lenses, as this gives you a 1.4 crop that you can apply to any focal length. Your 18-35 will perform as a 25-50 or 35-70 (roughly). I've mapped this to my front custom function button for quickly getting extra lens reach when needed.
Also , the Panasonic 42.5mm 1.7 is a very nice lens which can be found cheaply second hand. Much cheaper than the 1.2 and produces nice shallow DOF. Though I've found that the focus by wire sucks.