That is great - I have pretty much gone with all you suggested.
Just one question on the G2 Power plant and not the G5 - is that down to reliability?
As has already been said, it's simply that you shouldn't engineer a PP more than you
need to.
Also, bear in mind that various modules are disabled in that build.
The idea is, if you want to use the AFMU or mining laser, you'd drop out of SC, shut down the FSD and booster and
then power-up the AFMU or mining laser.
You could, theoretically, save even more power by shutting down the power distributor and cargo hatch
and shutting down the SRV bay until you land.
That
might allow you to apply a G1 overcharged mod' or, possibly, swap down to a 2A PP instead.
The main reason I didn't pursue this
too rigorously is that you can land on a planet, deploy your SRV (thus leaving your ship with the SRV bay powered-up) and your ship will then decide to launch back into orbit.
When you recall your ship it will arrive nearby and
then the game looks at how much power your ship is using.
At which point, you might well get to see your ship floating above the surface, completely dead in the water because there isn't enough juice to run the thrusters
and the SRV bay, and then it'll gracefully float down to the surface and explode.
In a nutshell,
never cut your power requirements so finely that you have to power-down your SRV bay to make your ship fly.
By all means, leave your SRV bay on priority 2 or lower but don't build a ship that
needs the SRV bay offline before it'll fly.