Re: Semi-beginner birder in M43, where to go from now (picture heavy)
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I can only advise what I would do and in fact have done. But first I want to point out that you already appear to have greater field skills than I.
I loved my 75-100 mk2 lens but it certainly has the limitations you mention.
Buying the Panasonic PL100-400 when it was first launched was the most expensive photographic item I had ever bought by far at that time. However I went ahead despite assuming it would be a 'one trick pony', but how wrong I was.
It not only captured noticeably more light (because it's razor sharp wide open through the full zoom range) it focussed much quicker (so I missed less fast small birds). And it very noticeably improved the quality of the captured image ... Especially at the long end. Plus it reached out to 400mm. So that was a win all round.
But initially there were a few problems (more on that in a moment) however it showed it's true potential when finally matched with my later purchase of an em1-mk2. Not only did my bird photography take giant leaps, the 100-400 is a great all rounder and stats on for butterfly's, dragonflies, flowers, air shows, jet ski racing, surfing and a multitude of other subjects too!
It's versatility of zooming from 100-400mm has ended up making it the best value of any piece of photographic kit I've ever purchased!
I won't get into a debate regarding comparing it's quality to the 300 f4 pro as I have never used one, but I will say that when bigger birds come close even zooming back to 100mm will sometimes only just fit the whole bird in.
* the one caveat that initially gave me poor results with it - solved when I bought the Olympus em1-mk2 camera - was that the Panasonic Dual IBIS system did not work well between my GX8 and the PL100-400, and needed to be switched OFF for sharpest results. However the PL100-400 fitted to the em1-mk2 allows a choice of IBIS only or OIS only, both of which produce really sharp images (this was an option previously not available to me). Using just one of either if those is the secret for sharpest results.
My now very old PL100-400 is also slightly stiff to zoom, but silicone polish sorts that out.
Best wishes.