I would suggest Org-mode via Emacs. Though it was initially designed as an organiser/to-do list, its use as a word processor is really good. Strictly speaking, it is a simple mark-up language closer to Markdown, AsciiDOC, ReText etc. It has a lot of capabilities from the simple markup (bold, italics, underline, strikethrough), to headers, footers, footnotes, tables. insertion of diagrams and images if needed. The syntax is typical of a simple markup language:
*bold*, /italic/, _underline_, ~strikethrough~, ^superscript, _subscript, [[fn: footnote]], etc.
It exports to *.odt, *.tex, *.pdf, *.md, *.html
In a sense the outcome is quite portable, and the file is essentially a text file saved as *.org which could be viewed with a command like cat or
cat file.org | less.
If you are used to WordStar in particular, then possibly WordTsar or WordGrinder would be of help.