Sunrace have several things in their catalogue that they could make but won’t actually make until a company places a large enough order. The aftermarket alone usually isn’t big enough to start the production line rolling.
If you are looking at keeping just one bike running, your priorities are quite different to someone running a bike shop. Grinding a tooth lower on a 1/8” 15t cog might be worth the effort to use an obsolete hub until an internal component breaks.
1/8” chain shifts just fine. Derailleur gears have been fitted to Sturmey hubs for a century.