maxis, celcomdigi also have but their plans are not so competitive and it might be bundled with a router with contract

yes and umobile have contract-less 5g plans for rm58 and rm68 monthly respectively (Yes 5G Wireless Broadband and U Home 5G)
yes limits you to 100gb if you use on 4g
umobile doesn't care how you use it

you can purchase your own smaller 5g router but it will cost quite a bit. 5g routers are not cheap
the portable yes one is probably their 4g plan, which would be slower

yeah you could, I think someone on the U Home 5G thread bought a cheap 5g phone to use for hotspot

but remember it's a data only plan, no calls or sms, just data