I also got my L1 this spring on a PML Tethys. And like you, the itch started for altimeters, dual deploy, L2..........
I got a Transolve P6K. If you can board solder - it's the cheapest way to get a solid altimeter. It has low resolution for altitude reporting (100 foot increments). But I like it. I would not recommend it if you are new to electronics. I made mine in about an hour and it works great.
For my L2 attempt - I modified my Tethys to Dual Deploy. I got an 18" payload kit from PML and built the altimeter bay into the supplied coupler. Drogue is deployed with motor eject from the main body and the main is with the P6K in the new payload section. I also used the PML ejection holders with the base glued to the top of the coupler/altimeter bay bulkhead. Works great!
I tested the altimeter by putting the main into the drogue bay and attaching a small chute to the NC and NOT tethering the NC to anything and using a smaller engine. (1000' foot predicted altitude) If the nose got blasted out of the payload bay @ 600' (altimeter main deploy setting) and returned on its own chute, my P6K worked. If it stayed in the payload section and returned with the rest of the rocket under the main, I screwed it up somehow. Either way my whole rocket was safe however.
My test was on a VERY windy day and I had huge drift on my 1000' flight enough to get it caught up in a distant tree. But my P6K worked perfectly. Nose popped out on que and returned nicely.
I never attempted my L2 flight due to the winds. I plan on using a Pro38 5 Grain. Hopefully mid August !!!!
Good Luck on whatever you try.