I suppose it's possible to hit a wall if your RNG luck is shit at least for the ultimate/soon to be ultimate+ fights, but in general, spending is pretty unnecessary. There's SSB fest banners coming up that'll be great for you to do x11 relic pulls on in search of a Hastega/Wall (Sentinel's Grim/Thyrus)/AoE cure AKA the Trinity so definitely save your mythril for that.
Sidenote: Bahamut SIN gave me enough MPOs to take Full Break to R3... fuck year.
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