Chibiakumas took 6 months to write, working every day for a minimum of 3 hours (and many full 9-9 days on weekends)
It would be easier on an amiga due to needing less optimization, and if you program in C or use prewritten graphics libraries it would save time too.
ChibiAkumas was sold through polyplay:
https://www.polyplay.xyz/
I wouldn't expect to make any money 'selling' retro games though - ChibiAkumas sold around 30 copies, at such a small number, I never made a penny - and I'm pretty sure PolyPlay didn't make any either due to the printing costs of such small runs, and the cost of 3 inch disks these days...
Your only chance of making money from retrogames is if you can convince people to kickstart you somehow (doubtful*) or if they'll back you on patreon
Of course, I'm only experienced with CPC/MSX/Speccy scene, maybe amiga is different.
* Nogalious by LUEGOLU3GO STUDIOS made their €25,000 goal, but they had to put around 3k of their own money in to avoid it failing, and the game had major marketing as I understand it was a passion project by a wealthy businessman