Baril floor mount Tub filler intallation help

[OLS G]

I'm doing rough in work for my bathroom tub and faucet.
The faucet is a floor mount tub filler which is mounted on a metal plate above the floor. Once mounted the connections to the water lines are under the floor.
(Brand is Baril, somewhat popular brand here in Montreal Area)
Image of the tub filler connection should be attached (if I did it right)
Out of the bottom of the tub filler faucet are a hot and cold flex line female 1/2 NTP connectors about 6 inches in length.

I need installation help please...
Supply lines to the tub are copper so the idea would be to install shut off valves in the floor (with an access panel from below), and run flex lines from the shut off vales to the tub faucet. The flex lines would allow me to run them up through the floor connect to the faucet and then push them back in the floor to mount the faucet.

Options:
the option offered to me at local HD store would be 3/8 toilet shut off valve connected to 3/8 female to 1/2 female flex line and use a 1/2 nipple to connect the faucet female line to the female flex line.

Concerns:
1) It's a big tub I'm concerned the 3/8 shut off will lower pressure and slow the filling of the tub.
2) My current option feels more like a hack than proper plumbing solution.

Suggestions please.
Thanks in advance.

 

[Terry]

You don't have much choice with those. If the female takes a 1/2" iron pipe thread, perhaps some PEX lines would be better. If you could use 1/2" ball valves and larger lines it would be better than reducing down to 3/8".