The first fork is whether you want a visual page-layout application or a code-driven publishing pipeline. Scribus is the desktop-publishing option that will feel familiar, with text frames, paragraph and character styles, precise PDF export, and color-managed print work on your own files. Vivliostyle and Paged.js take the other road, typesetting HTML and CSS into paginated books and print PDFs, which suits documentation and anything already stored as structured content. Teams that reissue the same publication every cycle often prefer the code route because layout becomes repeatable, while bespoke visual layout points back to Scribus.
Whichever road you take, expect gaps around InDesign's production polish. Deep behavior around parent pages, tables, GREP styles, footnotes, cross-references, books, and preflight profiles either works differently or has to be rebuilt. The real shift is accepting that imported files are starting points, not guaranteed replicas, and that printers may ask for more manual PDF verification than InDesign's preflight trained you to expect.
Migrate from InDesign while you still have access to it. Export each document to IDML, package the job to collect linked images and fonts, and keep a reference PDF for visual comparison. Do not treat a final PDF as an editable source. After import, inspect styles, tables, overset text, swatches, spot colors, and bleeds, then rebuild core templates natively rather than carrying years of accumulated document history into the new tool.