Gradually converting to AG...is this ok?

I’ve done several 2 gallon extracts and recently decided my Mr Beer LBK will do 2.5G just fine (with a blow off tube) anyway I purchased the ingredients for an AG Hefe (4.75# grains and .5oz hops). I just want to run my methodology by some pros and see if i’ll be OK…

Heat 2 gallons to 163° (for a 152° mash)
put my big @$$ bag in the pot, add grains/water and stir (make sure temp is correct)
put lid on and put in preheated (but cut off) oven
sit for 1 hr. (to stir or not to stir??? I’ve read both)
heat 2.5 gallons to 173° (total water at 4.5 G ----- accounting for 1G boil off and 1G absorbtion)
after 1 hr remove pot from oven, stir…it is at this point I was thinking about doing something i haven’t read much about…sparging the bag (mostly just read let it drain) but what if i get the bag out of the wort and siphon some of the sparge water over it for a bit? Wouldn’t I get better efficiency?
Anyway, in theory, I should have 3.5 gallons wort to bring to a boil for an hour.

P.S.- FWIW - i have a 5G and 3G pot, a standard un-modified igloo drink cooler, 3.5G bottling bucket and 3 LBK’s

Input is greatly appreciated.

Rather than messing around with the oven I would mash in the cooler, it will hold the temperature for an hour with little loss. There’s no need to stir the mash after initial dough in, you’ll just lose heat opening it up.

+1, use your bag so the valve won’t clog. I did partial mashes this way before I went all the way to all grain.