Three ideas.

First, are you taking photos of static or dynamic subjects? In other words, can you push your shutter speed down to make up for lost light? If yes, it doesn't really matter which sensor format you go with. As an added bonus, M43 stabilization is stupid good so you might even be able to skip the tripod.

Second, there's no beating a fast FF lens on a FF body but those are going to be big and expensive. Here's a quick comparison. The Sony lens is 1.7 pounds and costs $2,000 new vs the Oly's 0.9 pounds and $1,200 price tag. There are super fast third party E-mount options that will save some $$, but no weight (hi Sigma), and slower first and third party options that will save $$ and weight, but will also start to eat into the two stop advantage FF sensors enjoy in low light. For example, Sigma's 50mm f/2 lens, which weighs 0.75 pounds and costs $640, will leave you with around 2/3 stop advantage in low light. It's still an advantage, and it even costs and weighs less than the Oly, but it's not a large advantage in terms of dynamic range.

Finally, with a fast FF lens you're going to be facing a fairly narrow depth of field wide open. That 50mm f/1.2 FF lens will give you a 0.11m depth of field wide open with a subject that's 2m away. If you want the extreme background of your image sharp, everything closer than 35m will be out of focus. Conversely, that 25mm f/1.2 MFT lens will give you a 0.23m depth of field with a subject that's 2m away and with infinity acceptably sharp you'll have a sharp foreground subjects 17.5m and further away from the camera. Stopping down the FF lens will increase its depth of field, but will again eat into the inherent advantage a FF body has over a M43 body.

well, i don't own one currently, but pics i saw from the om5 looked good to me this article has some low light examples for the om5 and i cant remember it being a point of critic when i was reading up on reviews when i was looking for a camera, so i'd guess you'll be fine.

i actually bought a lumix s5ii, a full frame camera, and regret it sometimes a little bit due to the weight, bulk and price of some lenses and i'd probably buy a om1 in retrospect.