I think im able to get away with, a small defense (well i think its a bit small) mainly cause i always transport res daily, and never have above 1kk total sitting on a planet (less its my main fleet but thats a tad different )

400-800 mix of rl / ll
25-50 hl
10-20 gaus
20-50 ion
5-10 plasmas
1 ss
1 ls
20 abms

Though it be nice to even out my defense on all my planets, but i just hate investing any points what so ever into defense. its yucky

another way to raise defense is, rather go into vacation mode during holidays, try to connect your account 2 or 3 times a week, and use all your resources for building defense.
i did it this summer, and i haven't built any defense from that time (except a few for my newest colony)

Well i forgo that by not going into vmode

When your mines become very high you need some defense to shield the daily production

each of mine planets produces more than 2M resources a day
without any defense, everyone could steal me these resources