I tried something as below. So I have enabled the reward points for the order placement process. Right now the reward points are redeemable on the full amount of the order (Grand Total) and I want to allow redeem reward points only on the Subtotal, not on Grand Total.
So here my goal is to customers should pay shipping charges on order time and for that, I have modified the below file.
File: vendor/magento/module-reward/Model/Total/Quote/Reward.php (File overridden in the custom plugin)