Advice on estimating load and wiring a small cabin
The added up size of the breakers is not equal to (generally it's much greater than) the actual load.
There are code guidelines, or you can look at what's expected to be powered and what its actual amp draw is.
Tankless electric water heaters are foolishness, and the huge amperage requirement is one reason why (the lack of power savings is the other main one.)
Also you are apparently in 120V/240V land and don't realize that 2 20A loads on opposite phases at 120V are the same as one 20A load at 240V. That alone gets you down to 175A on your current list.
You also need a minimum of 2 20A circuits for kitchen outlets, and it's generally advisable to have at least one other circuit (15A is normally plenty) for the refrigerator and nothing else.
For a start, look at the actual running amps of your mini-split, .vs. its 40A breaker size. If you insist on the tankless foolishness, it's actual amp draw .vs. breaker size will matter as well, but it won't spare you much in that case. FWIW, that's one HECK of a breaker for a minisplit on a 288 sf cabin - I'm doing 1000 well insulated square feet with one that asks for 15 and uses maybe half of that.