You could keep small goldfish (well, they get to about 15cm and 25 years old) without needing filtration. No fish is a no-no as you then just built yourself a giant mosquito breeding place (!) And as you'd have almost no debris from big fish, you could leave the pond the size it is now! Just stop the whole filtration system. And don't worry about algae/pond turning green for a while: the fish love it. You wouldn't even have to feed them, they'll find their own food without any problems.
You'd still ideally have to put a net over the pond in fall (dunno if you have many leaves whirling around - they do sink and make a mess/produce methane etc on the bottom).
I had my pond going without filtration or skimmer for 5 years without any problem, with about 10 goldfish (don't take golden orfes, they get very big as well.)
Nets are cheap, and only need to be put/removed once a year.
It looks great, so maybe reconsider and indeed go "small fry"?
I gave away about 17 fish IIRC.
On a nice day you can always try to spend a few hours on your deck, feed them a few pellets at the time, and take pics when they come up. That way it's easier for others to see what types etc. you have.
Another way is throwing a half orange (peel & all) in the pond: they will play with it and eat from it (and it's healthy LOL). Again a good way to take a few snapshots.
If you love the layout, and don't mind just having a very few small fish, the least work would be a) sell the biguns b) feed pump directly to the waterfall (skip filtration system). A possible c) is - if already installed - keep a skimmer going, and empty that every week or so. Usually that's a small basket which will contain leaves. Not heavy, 5 minute job.
The BIG JOB usually is to clean out the filter. I don't know if it's brushes or mats or a combination etc, but it's literally a sh***ty job and even breaks my back. But with no big fish, there is nothing to filter, the volume is more than needed to keep "self-sustaining" (maybe add a few iris?/other marginal plants) so the little ones will be happy as ... erm... fish in the water. (sorry.
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30 grown koi produce a huge amount of crud. Remove them and your pond will stay just fine without "big maintenance". Done it, have T-shirt, it works (my pond is about 30,000 liters).
With that solution you would a) get money b) keep nice pond c) have little to no work. I dunno, take your time to think it over? Making it smaller would make it less healthy for the small fish (!)...
PS: I know a bit about types of koi, but not enough to "value" them properly. But for example, say, if you have a 50cm white one with only a red dot on its head then, yeah, that one may well be worth 4K or more...
So don't be a thief of your own wallet either.
Edit: I now see the pond was made using a liner (probably Firestone), so it is possible to "make it smaller" without having to buy anything at all. Just sell fish, drain, (doesn't have to be totally drained), peel back liner towards the part that stays, fill in the bit you want gone with sand/whatever, and "move up" the liner to it's new frontier - and cut away the excess. Sounds more complicated than it is.