I wanna +10 my Saizo so...

Rogue dagger + atk smoke?

Imo smoke dagger is more scarier. Because can target all stats, and your saizo is already debuffing attack, so thats superior than rogue dagger buff in defense.

If Silas gets demoted from the Nohrian banner, you could give Saizo his def wave. With that you can easily keep smoke dagger and profit from those huge debuffs.

I'm using smoke dagger CC Saizo on Gunthra's team and it's an absurd combo.
You could also consider Barb shuriken to boost his dmg potential. Saizo's damage is not high enough to take out tanky units, so he needs constant special procs. Barb shuriken allows him to use bonfire in every encounter even if he doesn't get doubled.

Fun fact, yesterday I was planning building up my Saizo too, mine is +df-hp, and Im planning to run him with:
Smoke dagger+(def)
Reposition
Bonfire
Close Counter
Quick Riposte 3
Even def wave 3(only if Silas get demoted)
and Close def 3 on the S seal
at +10 and summoner support he´ll have 51 with the even def wave active, and 57 in enemy phase, maybe giving him Rogue dagger+ and atk smoke could be fun

Rogue dagger is hard to pass up. With attack smoke like seeker mentioned, you get the biggest gains in tankiness - saizo gets +6 def/res, and the opponent gets -7 attack, meaning saizo's defenses might as well be boosted by 13 points, which is phenomenal. Of course, attack smoke doesn't debuff the individual opponent he's attacking, and he won't slow his opponents down like with rogue dagger. So hopefully he'll be quick enough to avoid doubles or tanky enough to take almost no damage. Actually you could double up with the speed smoke seal, that could be interesting. But also rogue dagger buffs his teammates' defense too, though I guess smoke dagger does a similar thing by lowering the enemy's attack. Idk. They're both good