the problem with being a larger entity is there may be legal requirements around your accounting system.
In many EU countries, if your turnover (total amount of money flowing through your business) is over a certain amount, or if you are incorporated (rather than sole proprietor) you have a legal requirement for double-entry accounting, and in some places a legal requirement to have an external firm do your accounting.
Yes, but once you are using Power BI you have lost the friendly interface the op described for categorising transactions, ie it is not a simple spreadseet any more.
I couldn't fit all of the transactions in a given month into Excel for my business. There are more than 1m rows. We are only an SME.