So hard to find underused vintage names that are usable in real life 
I brought up [name_m]Octavian[/name_m] with my husband, but he’s not having it! It’s a bit too much and too fanciful for his tastes.
I’m unfortunately not loving any boys names and our son has already been here for a few weeks, so I’m really not sure what to do now
Congrats!! 
Out of the 2 options you initially mentioned, I’d pick [name_m]August[/name_m] (due to the same reasons others have mentioned, i.e. the “Am-” beginning of [name_m]Ambrose[/name_m] & Amias).
However, I think it would be ideal to bring another letter into the mix, and give the new baby his own initial. I really love several of the other options you added in your longer list! Here are my faves:
[name_m]Edmund[/name_m], [name_m]Ambrose[/name_m], & [name_m]Otto[/name_m] (swoon-worthy trio!)
[name_m]Edmund[/name_m], [name_m]Ambrose[/name_m], & [name_m]Leander[/name_m] (dreamy, lyrical set)
[name_m]Edmund[/name_m], [name_m]Ambrose[/name_m], & [name_m]Wilfred[/name_m] (very classic & elegant)
I think any of these three options would sound amazing, & match beautifully!
(For me personally, [name_m]Octavian[/name_m], [name_m]Gilbert[/name_m], & [name_f]France[/name_f] don’t quite flow with the others.)
Cheers!
[name_m]August[/name_m] gets my vote ! I think [name_m]Amias[/name_m] and [name_m]Ambrose[/name_m] are too close in my opinion
I love [name_m]August[/name_m] as it goes well with your other names but still stands out. Whereas [name_m]Amias[/name_m] sounds a bit close to [name_m]Ambrose[/name_m] for me.
Some other names you might like are:
[name_m]Cassian[/name_m]
[name_m]Willem[/name_m]
[name_m]Ruston[/name_m]
[name_m]Jude[/name_m]
[name_m]Magnus[/name_m]
[name_m]Hugo[/name_m]
[name_m]Lucian[/name_m]
[name_m]Matthias[/name_m]
[name_m]Bennett[/name_m]
[name_m]Flynn[/name_m]
[name_m]Silas[/name_m]
[name_m]Ronan[/name_m]
[name_m]Solomon[/name_m]
[name_m]Anthony[/name_m]
[name_m]Ellis[/name_m]
[name_m]Miles[/name_m]
if [name_m]Emrys[/name_m] is too close to [name_m]Ambrose[/name_m], maybe [name_m]Ellis[/name_m]?
i love leander & lysander from your new options!! hidden gems for sure, and both go amazing with your set imo
some other suggestions (sorry for repeats)
edmund, ambrose & …
clement
casimir
isidore
lancelot
lazarus
laszlo
sylvester
orlando
bellamy
beckett
crispin
roscoe
pascoe
valerian
ulysses
donovan
evander
everett
felician
fritz
leopold
macaulay
percival
rochester
sterling
taliesin
viridian