Tried to use them, as many of mine expire end of this year - but... nothing available when I tried to search for flights. Maybe only for TG operated direct flights, but no connecting flights.
Note that the miles are only for the fare, but you still have to pay for the taxes, surcharges, etc. So in the end a mileage ticket from TG might cost you as much as ticket from another airline without using miles.
Good part of it however is that it's valid for a year. So if miles expire end of this year, you can book a flight in December for up to December 2024.
And yes - you can book online. Or TG office in Silom (didn't do that since before COVID so not sure that's still option).
And you can add up to 5 nominees - you can issue ticket for them in the same way. But if you want to change any name, it'll cost you.