are you MASHING base grains (2-row, 6-row, pilsner, munich, biscuit, victory, vienna) ? or STEEPING specialty grains (black malt, cara*, chocolate, roasted)?
mashing at 153/154° is good, but I don't know if 30 minutes is long enough for conversion. higher temps (156°) for shorter times (45 minutes) for fuller body & lower temps (148°) for longer times (75 minutes) for lighter body
steeping at 154 for 30 minutes is good.
a "partial mash" is any amount of those base grains mashed to conversion with extract added later, in the boil
steeping specialty grains and adding extract is still only extract brewing
that said; using your method, mashing for 45-75 minutes, or steeping for 30 sounds like it will work and the beer will be tasty and good. the only limitation is the size of your cooler
"Beer. Good." - Words of House Grog
drinking: Otto M. Gourd Pumpkin Barleywine, Jewel Thieves Apple Wine, Fresh Squee Zed IPA - bottle conditioning: LoCo Foot Barleywine, Basque cider - lagering : Schwarzbier - fermenting: apple wine, Skeeter Pee - on deck: Grodziskie