My current leveling skill right now is Overbrand, due to that I put a Hillslion card (ATK +25) since AS increase just a portion of it and AK wont increase its damage if I'm not fighting Boss class monster. Putting more FS enchant is also a good idea.
Imperial Guard is a shield specially designed for RG. You can put few racial card in it since its cheaper than Valk shield. But if you can afford buying 10 Valk shield and put every single racial card in it, then go ahead. You can also consider getting a Valk shield + Hodremlin card for universal use.
The main reason I like Imperial Guard is because of Shield Press damage buff. I can do 6-8k with my lvl 3 Shield Press which is higher than my lvl 5 VP(4-5K) but lower than my OB(18K~). Plus they got higher def compared to valk shield (120 def vs 80 def) but Valk shield is better in term of elemental resistance.
I dont have 120 STR yet but I wear Temp STR with FS7 & Muscle Fool enchant. MF can give +1000 def when it proc and FS7 gives you more ATK to kill the mobs faster.
Compare that to a variant boots, you will take more time to kill your mobs and take more damage. Plus HP/SP consumables doesn't seem like a problem to you, then why not go full offensive?
Btw my RG also wears HBP too and my stats are build based on it.
And I only wear variant boot when I'm in mid TI and someone needs my Sacrifice skill (defensive mode).