I use second runnings / Parti-Gyle as often as I can. But as my primary mash efficiency rises theres less and less usefulness in the parti-gyle. As for me using the grains, it's compost or given away for chicken feed.
If your mash efficiency is still below 85% there's still plenty use in the parti-gyle.
I could usually get 5 gallons of 1.025-1.030 parti-gyle wort from a 12 gallon high gravity primary brew mash.
Slow Fly sparge for primary keeping mash water 1inch above grain. When primary beer is done sparging, stop the fly, stir and let the parti-gyle rest. When done with your primary boil, then batch sparge the parti-gyle wort.
Anything bellow 1.020 isn't very useful by its self. Most uses require adding a little DME, grain steep or sugar to bump it to the range you want.
Parti-gyle wort is low gravity and has many historical style uses like scottish-60.
It's great for starter wort. I boil and adjust to 1.040, Then refrigerate and strore in 1 gallon jugs.
It's great for malted ciders /light graffs, I find it even eliminates need to backsweeten. Especially if you steep in malts.
Don't mash out instead keep sparge around 154°, if you intend to allow the second mash to rest.
Adding a little more grain to second mash helps kick start a second conversion rest.
Add adjuncts to boost gravity.
Boil down to increase gravity. Usually pretty easy to get a fast violent boil since its usually 1/2 your normal volume.
Press the grains as the parti-gyle volaufs. I use a potato masher, works pretty well.
All in all how useful your parti-gyle is depends on a lot of factors and what its used for. I've only touched on the processes and uses as an overview.