Hello,
great to know someone breeding stag beetles. It's better to carefully dig out the larvae (grubs) at L2 to L3 stage and place each grub in separate deli cups with lids (either 16 oz or 32 oz). Make small pinholes on the lid. Substrate should be kept moist but not soppy wet. All the substrate has to be heated in oven at 450 to 600 deg F with plently of water poured onto substrates before oven. Heat it for 90 minutes.
Femake stags lay eggs throughout there breeding life for about 2 to 3 months so you may still find some eggs and L1 larvae mixed with the L2 and L3. Just be very careful teasing the substrate away with balsa wood chopstick. A balsa chopstick comes stuck as pair so you need to pull them apart and just used a single stick to dig the substrate very slowly. I spent few hours to dig out my 167 grubs few months ago. they are now mostly in L3 stage looking juicy and plump.
For the logs, I use have decayed (but still looking light color inside) hardwoods. I prepare my own substrates and logs. If you find a log that feels light for it's appearance, then it's decayed. Then make sure the inside is more light colored. For final check, drop the log from your height to a cement sidewalk to see if it breaks into 2 or 3 pieces. Make sure it doesn't crumble into small pieces when you crush with your hands. Then it's the best log for your Adult Stag breeding. Grubs also like such logs as well as slightly softer logs that crumbles in your hand with slightly more force. However, the inside still has to look light colored (white to creamy color).
By contrast, adult Rhino beetles don't require any log to breed. Just need a very fine compost soil. The Rhino beetle grubs prefer substrates that consists of very decayed soft dark reddish brown wood that easily crumbles in your hand with slight force.