I've been thinking a lot about this the past couple of days but ultimately decided to get a strobe, even if I pretty much only take pictures of very slow moving critters..
The MF-2 seemed to be one most often recommended but I couldn't really find it used in the EU (I wish ebay worked well here but it's almost all about US, Japan, UK..). Instead, I bought a YS-01 for a decent price. Looking forward to trying it out on the next trip! Also got a tray, trigger etc.
I might be able to get a good price for a used old sony a6000 with underwater housing, but then I'd have to purchase a proper lens and thread/port for macro (as I believe the current owner have done wide angle photography only), so it might still be a bit costly in the end. Also, it wasn't clear to me if it would get close to E-M10/R100/R50 image quality. Any ideas?
In any case, thanks everyone for all the help you've provided!