I think if you are operating on a limited budget then this is a) overkill and b) not cheap. Hey ho.
we are focused on saving costs
Redundancy costs more…
I can honestly say that in the several years that I have run my servers the only downtime has been when I have updated. That’s it. It has been reliable and robust.
I’d spend my money on decent reliable hardware and have some good backups - I think you are chasing some ghosts here.
Not really I’m afraid - you have not got a cure and I don’t believe arbiter/sharding is supported.
Ah yes - note this has been covered many times before. You need to search thoroughly.
So sharding isn’t, and won’t ever be, supported.
If you need redundancy that badly then invest the money and run a couple of Mongo servers which is supported.