There's a lot of interesting stuff in the time-nut world. There's a fun mailing list at https://time-nuts.febo.narkive.com/ and Jeff Geerling has been doing a lot of stuff with clocks lately on YouTube. An interesting intro project is to set up a GPSDO and track the long-term aging of an OXCO or Rubidium standard (many of which are surplused out of telecom applications for somewhat reasonable prices on eBay).

Also vaguely related, the precision clock kit by Mitxela - https://mitxela.com/shop/clock4

Then there's the EFOS3 Hydrogen Maser by https://www.efos3.com/

There is a very deep rabbithole in here.

Don't be surprised if over shorter time spans your OXCO is more stable than the GPSDO (and the Rubidium standard will almost certainly be more stable over the mid term as well, you'd have to do a fairly long run to see any measurable deviation at all).