You may have to trade off the smart modeling of PianoTeq for brute force multiple samples to get the feel of a piano. A Pi 4 with extra RAM may expand your options. The smallest samples you find are pitch shifting a single sample, giving sad results. (I am assuming there are suitable SF2 players for MOD)
Some free SF2, SFZ (Ogg compressed) samples are pretty good. (I tested a few of these on my desktop computer with Sforzando SFZ)
(You might have to settle for the one .sf2 offering of the SalamanderGrandPiano, to play on a Pi “not included, and in particular: amplifier velocity tracking (amp_veltrack), noises of pedal, hammer and string resonance on key release.” the 310Mb .tar.xz archive extracts to a single 1.27 gig SF2)
Looking a little deeper.
I examined the external files of an equivalent SFZ version to the all in one SF2 package’s hidden content.
There are a total of 641 samples, including 16 separate velocity samples of each note (giving the nice dynamics) , there are also some mechanical sounds like a .455 sec. key release sample for each of 88 keys, a couple of pedal down and up sounds, there are also 3 sets of 23 fast release samples, that appear to devide the 69 active notes into 1/3 ranges. “HarmL,…” “HarmS…”, “HarmV…” "hammer and string resonance?)
All 41.5K, 16 bit samples. (the files could by perhaps 1/2 the size if they pared down all the zero volume decay periods)
A backup source.
The smallest sfz, (84Mb unzipped file) option for: SalamanderGrandPianoV2_OggVorbis compressed.
Has good velocity dynamics (missing the bottom 19 near sub audible notes of the scale, no aftertouch or mod wheel.)