. . . However, I have most recently played on a Yamaha keyboard with a decent piano sound, and I absolutely do not like the sound of the Casiotone acoustic piano - it sounds extremely digital and nothing like a piano..
. . . Which model Yamaha was that? It would give a benchmark for what you consider "decent piano sound".
It's very difficult to get decent piano sound from small, light loudspeakers and amps -- and that's what you have, in the CT-S1. I haven't heard one, and I can't judge the quality of the built-in piano samples:
. . . I suspect the reviewers who are enthusiastic about the sound, are listening to it using headphones or outboard loudspeakers.
So, _within the limits of the built-in piano sounds_, your alternatives are:
(a) The amps are only 2.5 watts per channel. The loudspeakers are 5" (according to specs), but the housing of the CT-S1 doesn't have enough volume to get good bass response from them.
There's a headphone jack on the CT-S1. A set of good headphones (or good earbuds -- Shure SE215's or similar) would give _much_ better sound than the built-in amps and loudspeakers.
(b) It's hard to find small, light loudspeakers that will do justice to piano sounds. I can't find any "Acton" loudspeakers. Roland makes several small amps in the "Cube" series. The CM-30 might do what you want. It's 12 pounds, which isn't super-light.
I've been impressed by the 3" Mackie CR3-x "studio monitors" -- they produce good sound for their size, weight (8 pounds per pair) and price. They have no grille over the speaker cone, and you'd have to fix that, for travel.
If the Marshall you're looking at is a "guitar amp", it will probably have a limited frequency range. That may be OK for you.