Thanks for correcting me, I have updated now. Yes you should NOT put tax line on vendor / customer line items.
It does not really make any sense to load balances in Apr, as you are anyway going live in June, you can directly load May balances.
Do you have any bespoke program for journal upload? Otherwise, manually entering would be a huge task. Please ask your ABAPer to create a quick BDC for you.
Otherwise, follow this: