I can see the 30 minute limit being a problem in contexts like that... If you're quick and you're paying attention, you can just hit the record button again, but there'd still be the risk of losing a moment. Worse if you ever have to swap batteries... which is why I kinda wish the E-M5 MkIII had a battery grip option. Maybe the pinout of the connector will be able to support something aftermarket? I dunno, I'm not sure there would be much demand.
Yes, two of my major gripes with the E-m5 mark III is the 30 minute video limit, and not being able to power the camera with external USB-C power directly.
But the normal solution to powering cameras like the E-m5 mark III is to use a dummy battery with a cable that snakes out to an external power source. Unlike Panasonic, the Olympus battery cover does not have a hole for the external power cable to fit through, but generally you can take off the battery cover.
I don't (yet) have the E-m5 mark III (*), but other Olympus cameras I've used are fine with a dummy battery that feeds them power in the 7.4 to 9 volt range. BLS-1/5/50 batteries use a normal 2 cell lithium-ion battery, which is 8.4 volts when freshly charged, and rated at a nominal 7.4 voltage. BLN-1 batteries use a slightly different chemistry and have a nominal 7.6 volt output. I imagine BLH-1 batteries are probably similar to the BLN-1 batteries.
Olympus doesn't seem to make their cameras work differently with batteries that identify themselves as 'official' batteries, so you can supply any acceptable power to the dummy battery.
Having the cameras accept 9v makes it a lot simpler, since 9v is a common voltage for external power.
If you have a 3D printer and some familiarity with a soldering iron, you can make your own dummy BLS-x battery:
Or you could just order a dummy battery from Alibaba:
Here is a dpreview post I did about powering my Stylus-1:
Now, Panasonic is somewhat different. First of all they actually do make an official external power kit and their cameras typically have a hole or a slot for the external cable to fit through. In the case of the G85, the official kit wasn't available in the USA, but if search out on the inter-tubes, there are various third party suppliers that make the dummy batteries. Panasonic also looks for a battery that identifies itself as 'official'. If you don't have an 'official' battery, the camera will only accept voltages at 8.5 volts or higher on the assumption it is external power (and it disables the battery meter and I believe turning off the camera). The second generation of third party dummy batteries now include the proper resistors so you can feed it 7.5v and the battery gauge will work (using if you are using a larger 2 cell lithium-ion battery).
In terms of the show I recorded last night, I did put grips on both the G85 (video) and E-m1 mark I (stills), but I still had power in the grip battery after the show (1.5 hours total), but the meter was on saying lower power. I do like having the grips, and on both the G85/E-m1 mark I, I set the camera to use the power from the grip first and when the rear display indicates it has switched to the body battery, I can easily change the grip battery. I've done it on the G85 while recording video, and as soon as I inserted the grip battery, the camera switched back to using the grip battery.
* I am planning on buying the E-m5 mark III, but my salesman at my brick and mortar store has been out sick.