Awesome job, Mainer!

You say you are excavating in clay, yet the pond keeps filling up with groundwater.

Clay is almost always an impermeable material. Something in your subsoils is connecting your pond to the groundwater aquifer. I am only asking so you get your final construction finished as you want it.

Where is the groundwater level in your pond excavation when you have taken time off from the seat of your excavator? Does it rise to an elevation that you would be happy with as the normal pool water level of your pond? If so, then you should probably leave your sides and bottom unsealed and keep it as a groundwater pond.

How much does your groundwater level vary between wet years and drought years? On my farm, it varies by over 15 feet! I cannot operate a groundwater pond as a managed fishery under those conditions. However, I expect the groundwater table may vary much less in your area. If so, then that further supports you choosing to have a groundwater pond.

IF, a groundwater only pond will match your needs, then you might consider excluding the surface water from big rain events from entering your pond. That water will bring silt, leaves, and potential thermal shock events to your pond. All of those are big negatives. You could easily ring your pond with a shallow berm based on how you are moving your spoils.

Finally, have you considered a small pond upslope of your main pond? Lots of people on the forum do great things in tiny forage/grow out ponds. If you are allowing surface water into your main pond, then you could use the upslope pond as a silt trap. Much easier to fire up your excavator and clean that out to full depth every few years, compared to slowly losing depth in your main pond.

Good luck on your fabulous DIY pond project! I am just throwing out some more ideas before you complete your finishing touches.