Hi, are you at the end of fixed rate now?
AIB cashback is a fixed amount regardless of balance. So pay off lump sum as soon as you can. Switching process could take around 3 months, so better to save interest for that period.
If you pay off lump sum, with a remaining balance of 150k, you'd need to reduce term to 10 years for repayments of 1500 per month. For a fixed rate mortgage you'd need to reduce term first and then fix. That's a permanent adjustment to your agreement. If you ever wanted to reduce repayments in future, it would be at banks discretion. (You can of course pay off lump sums on a fixed rate, subject to bank calculating if a break fee is due). If reducing your term and balance that significantly, gave a look also at your mortgage protection as you should be able to reduce cost of that.
Or fix for 12 months, and pay off any extra you've saved up, then fix again.
With a variable rate, you could keep term at 25 years, but pay extra whenever you want.