Many thanks, Anton, for your quick answer & this short and already informative piece of text (without consulting the links yet)!
Looks promising enough to spend a few hours on it and even to acquire an appropriate development kit in order to gather some real world experiences.
Perhaps you'd put this text (slightly modified) in a sort of github wiki page to also allow others to get a quick insight of scalability & memory resource indications.
PS - prior to sending this response I scrolled through the webserver & libmodbus article. Looks very interesting. I'll take this as an starting point for first experiments on STM32 platform. Up to now, I mostly work with NXP MCUs (& formerly also TI/LM3 & M4F), but some peers have ever incidentally indicated that STM32 is also good stuff.