Congrats! The A7III is the reigning King of low light with an almost unfathomable reservoir of usable clean gain (ISO).
If you play that incredible ISO to your advantage, you can literally walk fearlessly in the night with a cheap kit lens, or a small APS-C E-mount prime. (or tiny M-mount pancakes from voightlander if you don't mind a some vignetting. 35mm, 40mm and 12mm and really tiny. )
For small but fast manual primes, SLR Magic's range of FE cine primes are also significantly smaller than any autofocus FE primes.
For shooters coming from M43, the full frame DOF of the A7III will be nothing short of gobsmacking. An f4 lens FE lens will deliver similar DOF as an f2 M43 lens.
If you did your maths and realise that one zoom is lighter and cheaper than three primes, then consider the new Tamron 28-75 f2.8. Definitely a bokehlicious mount and forget lens.
If you need a little more reach, Sony's 24-105mm f4 OSS is the lens of 2017. (Think of this one as a 12-50mm f2 in M43 terms!!!!)