Starch conversion. Is my mash working ?

I just partial mashed
4 lb grain
1 lb specialty grain
1 lb flaked barley

For a stout.
I added 5 lb extract.

I got OG 1.036 which seems on target for 5 lb extract...but it seems it should be higher with the converted sugars.

My mash looks like this.
Heat water to 160F, add to a plastic cooler to preheat.
Dump preheat water.
Add grains and 7.5 liters 163 F water.
Then check temp...too low. I wanted 150, I got 130. Put mash in pot and heated to 159 F. Put back in cooler, let sit 90 minutes. Temp dropped tp 155.
Added it to the wort. Added extract and hops, etc.

Tried a starch test but was inconclusive. The stout beer was too dark to tell if there was a color change.

And can anyone give me a better system for using a beer cooler as a masher?

How did you separate the wort from the grain? Did you sparge? When you say you "added it to the wort" are you saying that you made up wort in a separate container with water and extract?

If that's the case, you're doing it wrong. You want to create as much wort as you can handle with the mash/sparge and then add the extract to that.

By heating to 159F you completely destroyed the beta enzymes, which made for a poor conversion. It also sounds like you did not sparge the grain. This cost you half of the sugars that did convert.

Partial Mash Simplified - Home Brewing Wiki

OK, this is making sense. So it seems that I should start my mash out at 150 F to covert the low temp sugars, then add 180 F ??? water to raise the temp to 155ish to convert the higher temp sugars? can I add boiling water to raise the temp without ruining the mash process???
I mash for 90 minutes. How's that sound?
I just ordered some grain sacks to contain the grains in my mash cooler..and create more of a mash tun.
And to asnnser the other question, Yes I am sparging. I simply pour the cooler contents through a strainer and sparge with 168 F water.[I now know I can use 180F]. Then I add it to the wort pot and add the extracts.

Pouring water through grain that is sitting in a strainer is a really low efficiency technique. Add sparge water back into your mash tun, add the grain back in again, stir well and separate. THAT is batch sparging.