Hi Дилян (it's been 30 years since I briefly studied Cyrillic, forgive me for copy/pasting ;-)
Your logic sounds good to me, and I gather the use cases in HyperKitty are different from those in Postorius. So I encourage you to continue work on Postorius and its dependencies (I think that's just django-mailman3), but be careful about proceeding on HyperKitty.
As someone whose "roadwarrior" notebook is a 2016 Mac and primary work tool is XEmacs which hasn't done anything really new since 2010, I do want to be sensitive to people still using old browsers -- if there are any, I bet you'll find them among Mailman admins! I would appreciate it if you keep notes about where there are changes that do NOT have no-javascript alternatives. Abhilash has been doing some work about making the Django apps more no-javascript friendly, so I don't think worries about extreme backward compatibility should stop this kind of modernization.
About the "size of change" estimate, when you do have a handle on that please post here briefly about that and any other interesting aspects of the work.
Steve
-- GNU Mailman consultant (installation, migration, customization) Sirius Open Source https://www.siriusopensource.com/ Software systems consulting in Europe, North America, and Japan