Testing an AR750S (“Slate”) here, with the v3 -testing build from January 28th, I see a peak current draw during boot of 520 mA with no wired connection, but wireless enabled (with no clients). It idles around 410 mA.
This is significantly higher than my AR300M-Lite. There is a CPU difference between the two units, with the AR300M-Lite using a QCA9531 at 650 MHz and the AR750S using a QCA9563 at 775 MHz. The AR750 (no “S”) uses the QCA9531 at 650 MHz and probably has lower power consumption than the Slate due to the slower clock rate.
Adding an Ethernet client on my AR300M-Lite added about another 40 mA of idle draw.
Do you have control over both endpoints, or are you “stuck” with L2TP at the other endpoint?
If you have control over both, I’ve had good luck with GRE tunnels, both L2 and L3.
For unencrypted L2TP, L2TP Client: OpenWRT - AAISP Support Site looks to be a good reference, though I have not tested any of it.
Note that
option _orig_ifname <anything>
option _orig_bridge <anything>
are remnants of previous LuCI-based configuration and can be safely deleted/omitted.