First of all, the difference is 1746 matk at T10 between Staff and Rod.
My argument isn’t weak. You have 19.98% less magic attack compared to Staff.
If we consider that everyone can ± reach +11 with some golden anvils if needed, you have to cover 2861 matk.
To reach a similar magic attack power you have to upgrade your rod to +22 or +20 and add a 9 star blue gem on top of it for your shield.
However,that’s not all. Let’s say we include staff and rod stats as well. The best rod for an ichor is probably Wastrel Rod since it has +74 INT and +232 matk.
For staff it’s probably Masinios Staff with +122 INT.
In Re:build, 74 INT translate into 148 matk + 35% of your base matk (~390 at lvl 390) +232 matk = 516 matk.
The 122 INT from staff translate into 244 + 60% of your base matk = 478 matk.
That’s 38 matk in favor of the rod,though it’s not closing the difference any further.
To make a real difference, you have to get another ~50 INT from a shield ichor (50 INT = 197 matk).
Adding both together, you get a difference between staff and rod+shield ichor of 235 matk.
Let’s say we have that 9 star blue gem as well, that’s 342 more matk via the rod and shield combo, leaving you at in total 577 extra matk in comparison to the initial 1746 more matk of the Staff, meaning after all these extra investments, you’re still at least 1169 matk short to reach the matk value of the Staff at the same enhancement level, which is ± equal to the difference between +11 and +20.
By the way, about the enchants via jewels: Nothing is keeping you from having both weapon damage and boss damage on your equipment. You can have as many offensive stat boni on your equipment as there are spaces to apply boni (basically weapon damage, boss or PVP damage and crit rate, which is 3 of 5 enchants you can have in total with a two-handed weapon).