Any advice on this one?/ Sounds like iOS users are small in number, but I’d like to continue using without needing plugins (smallest possible foot print).
The n-Track Sampler is used by any sound you select in the Instrument browser. If you mean you want to load a custom sound to use as instrument or a custom soundfont file, that’s not currently possible with the n-Track Sampler on iOS.
Recent versions of n-Track Studio for iOS allow importing custom soundfonts, just add a new instrument track, then scroll to the bottom of the left column of the Instrument Browser and select ‘Add custom soundfont’.
You can of course also 3rd party AUv3 instruments including Koala.