If we plan to move house right after MOP, should we repay our current HDB loan quickly or slowly?

Depends on what you do with your money.

If you trust special account, some people transfer money there to earn higher interest.

If you can earn more than 2.6% interest with your cash, you can not repay.

Do note 1st 20 k of your OA is earning 3.5 % interest so no point lump sum pay down using that money.