The best value for money proxy I’ve found for setting up slaves is @HenryCooper raw mobile tier - $20 and the average user seems to put 10 accounts on it.
When a slave has considerable growth, do you upgrade it to it’s own 1:1 proxy to avoid the risk of another account on the raw tier getting banned and being linked to it?
They won’t be linked, if you don’t overload it. Yes HC proxies are medium good (aside the fact when cleaning lady plug them off ), 
No, it’s no need to put it 1:1, you can leave it as they are
You can easily stack up to 20 or more accounts if you know what you are doing but that required a bit or try and error . General speaking I would on stack as far as you can economical handle it.