Personally I would dig out your area and if it is as you say clay like material then I would tamp the earth with a handmade wood tamp.
Or you could buy a metal tamp. That will compress the soil and create a natural bed capable of holding water long term. If once you try tamping and the bed of your pond still seems porous or brittle you would need to add something like clay, which you could find and dig up and transport to your pond. Mix clay in with your existing dirt and create a compressible layer. Then tamp that down. If hand tamping is not your thing you can rent a wacker packer, fire it up and bang it around your pond.
If that doesn't hold water still you could buy some cement mix and add it to your material. Cheaper than that fancy stuff you were mentioning.
Really it boils down to budget in the end. How much you want to spend versus how much labor you are willing to expend.
Good luck!

Hello everyone, I’m not sure if people are still visiting this thread but in case anyone does come around with similar issues I wanted to update. My zucchini are looking better than anything else in the garden at the moment. The weird transplant effects have been replaced by lots and lots of leafy greens and beets being chomped on.

I kind of just waited to see what would happen to them, not addressing any kind of possible fungus issue. So I think that those who mentioned transplanting issues/cold damage may have been right, because they’ve kind of just bounced back and are doing great. Like said, better than everything else I’ve planted.

No fungus issues! Yay!

Awesome you guys, thank you so much for the insights! So happy I decided to come on here and ask before jumping into the project.

I will continue to study your posts, a lot of new knowledge there for me, which is great! I think I will have my dad (construction worker) read your posts and help me interpret and then hopefully come up with a plan.

Until then, thank you all so much for your guidance!