On the HS codes, you don't have to key them in one at a time. Xero lets you bulk-import inventory items by CSV, so the quickest route is to export your product list, drop the commodity code into the spreadsheet (in the description or a custom field), and re-import the lot in one go rather than editing 1,000 items by hand. If you've still got a master product export from Sage, add the codes there first and it becomes a single upload.
Worth being realistic though: Xero doesn't hold HS/commodity codes as a proper structured field the way dedicated stock systems do, so if customs paperwork is central to what you do, some people keep an inventory or commerce app that holds the codes properly and let Xero just handle the accounting side.
For the move itself, if you've not started yet, a conversion tool like Movemybooks will bring your Sage balances and up to 24 months of history across, which saves a lot of re-keying. Pick a clean conversion date (start of a VAT quarter or your year end) and run Sage and Xero in parallel for a month so you can reconcile opening balances and the first VAT return before you switch Sage off.