Your comment on GPS and transponder is incorrect. Here is the reference:
https://www.faa.gov/about/office_or...rps/2016/Nkugba_2016_Legal_Interpretation.pdf
My IFD-540 has a permanent clock display. Maybe other GPS units do not.
It is not incorrect. You're to be forgiven for your confusion on this, and you're in very good company, but if you follow the "IFR Clock rabbit hole" to its conclusion you will realize you've misunderstood the Nkugba interpretation, which doesn't contain the support for what you've inferred.
It's a long thread, and it starts with a different question, but here it is:
https://www.pilotsofamerica.com/community/threads/bad-tach-should-i-go-gi-275-eis.128365/
You could start on or near post #30 if you'd like to save a little time.
I am not intimately familiar with your IFR GPS. If the installed appliance's time field (top of display) is configurable and removable, it is not suitable for use as an IFR clock. If it is indeed permanent and not user configurable, it
may be suitable. Usually, it will be full panel suites (G1000/etc.) which contain permanent clock displays. The GNS430/540, GTN650/750, etc. do not have them. You may own an exception to the rule, but "most" of the Navigators out there do not meet this requirement. Possibly, yours doesn't either, but I won't give an opinion on equipment I'm not familiar with.
On the other hand, you're definitely in err regarding transponder displays. There are no transponders I'm aware of which meet the requirement for a permanent clock display. That's a no go.
You can follow the thread I linked to for the rest of it. I don't have the time to go through all of that again, but the ability to navigate the guidance to come to the correct conclusion on the subject of "legal IFR clocks" has piqued my interest as a possible FAASTeam webinar. It is highly technical and nuanced, but I think there are takeaways for the end user vis-a-vis how to parse seemingly conflicting guidance.
Best of luck,