6 to 8 seconds is quite slow - Agreed
A max 3 Sec load/save time is about the slowest you would want to go without all the users wanting to kill you. The errors you are experiencing are varied and not immediately connected. (Cravat I have never seen frappe/erpnext dealing with a workload this size. And its an old version.)
Possible Ideas?
1: I would keep the current financial year. - 365 days is nothing. Retail shops often “cash up” at the end of the day. So treating accounting on a per day basis sounds like it helps. Then enter it daily into a 365 day second master frappe install.
2: Run two copies of erpnext. One purely cashiers, One Back end. Automate after hours one way cashier to back end sync.
3: Check this discussion out. @ganas is doing this currently. Looks well thought out.
Supply each cashier with a copy of this on a tablet. It only sends cashier data. Could limit some sql traffic. Have a full install of frappe for back end.
V9 is full of very cool features and is progressing very rapidly. Some features are diamonds in the rough/unfinished. Once a page/module has been opened once that day they reload fast. Good caching somewhere. For you I don’t see any option but to spin up a V9 instance on one of your many servers and test it. Can’t be much worse than now. Yes upgrading could be a major PIA. But maybe not??? But probably yes.