IMO - in_checkout orders should just be skipped, in Ubercart any in_checkout orders without entries in uc_cart_products are essentially "lost" to anybody except admins. There's no way for a customer to really complete a checkout IIRC with just an in_checkout order...
Even the review page in Ubercart pulls product data from the cart, and just prior to the review page the order product data is dropped and replaced by the cart contents with every submission. In fact as a rule of thumb you could say the order is write only during checkout, it's never really read from as a data source to generate any of the steps.
All that to say, if uc_cart_products is migrated, one can safely drop all the in_checkout orders without losing anything of value, there's really no reason to migrate both.
That's because as you suspected your local PHP binary in the system path is 7.3, you can either change this to the 8.1 version (not sure what distro but you can search how to use the utility update-alternatives for a handy solution on that front), or more easily just pass the argument:
--ignore-platform-reqsto composer. This will ensure it doesn't bother examining your local system and will leave that compatibility determination on your plate. You can also call the composer.phar archive with the 8.1 binary by doing something like:
/path/to/php8.1 composer.phar install