Presuming the new 5 year digital nomad visa isn't in place soon, or maybe at all, what I seem to be looking at is this:
KITAS Social 6 months... 1 extension... 2 day(ish) visa run and reset the whole thing... with the potential risk of them changing the rules on it and me not being able to extend?
Until such time as I'm 55... then can go down the retirement track, or decide to become an investor/marry a local/etc.
based on my understanding which is far far from the gold standard, the above info might be a bit muddled, I dont think there is a KITAS Social as Social usually refers to the B211[a] (in my experience) visa (VITAS class) which comes in many flavours; tourist, investor, social .....
Without being sponsored by your employer or a local legit business, owning a business PT-PMA, being an investor or marrying a local I do not belive that any KITAS visa will be available to you.
Visa runs are also currently a thing of the past - unless you want to - as on-shore applications are currently available, a breeze, and not onourously challenged.
So I'd break your plan into two parts:
1 Arriving on the island
2 Staying on the island
For (1) you can either arrange, in advance, most probably via an agent, an offhsore 6 month B211A which will require a sponsor, either business or personal (depends on the visa sub-class) regardless as to whether its an agent or a personal connection... or you can arrive on a 1 extension VOA issued at the airport for 30+ 1 extension (30) days and migrate onto a B211from there locally - VERIFY this as I believe it but have not done it. You shoukd regardless fire out some mails to some of the local visa agents getting quotes and proposals.
The B211A, regardless of sub-class should now come in 2 flavours; a 2 month + 2 times 2 month extension (=6 month) version or a full 1-shot no extension 6 month version. A month or so ago the 1-shot version was legal but the immigration processes were not in place to support it so it wasn't actually possible.
Currently (and of course always subject to change), once on shore you can make an on-shore application for further 6 monh B211A visas subject to the good graces of immigration who are currently quite happy to approve them.
This means that at the end of your 6 monnth offshore viisa you shoul have no problem applying for successive on-shore visas so long as you keep your nose clean and dont do anything stupid. This can be done either with an agent or a friendly local deemed eligible to sponsor.
Most importantly you need to note the B211 (social or otherwise) is a 1-shot visa and if you leave RI it expires.
So assuming you do a bit of regional travel and head home now and again you can probably muddle through till the 5-year gonad
or the retirement vissa becomes viable or you get a temrinal case of "local fever".