The new Class D GAN1 amps are usually small. I am using a modded PeachTree GAN1 on a great speaker and it sounds amazing. Though I cannot recommend it to you. Check out other GAN amps (just not the GAN1 since it won’t work for you). I have not heard the AGD amps, but people seem to love those GAN amps and they are tiny.
The Benchmark AHB2 is one of the best amps and also tiny. It does not have massive amounts of power but if you speakers are not too demanding it could work great. It is the most neutral and quietest amp around so you can flavor with source and preamp.
I also have the same situation as you with regards to an amp to replace the GAN1 (I need more ease of use). The CODA #8 is 14 inches deep and works for my Ikea ’shelf'. I think I going to get this amp (again).
I was also looking at the smallish NAD M23 (Purifi) which likely sounds like the AHB2. Though I have concerns about reliability from reading posts here.