Okay, reading over everything, including "cost no object", "wooden structure will have minimal flex", etc., my vote would be for West Systems Epoxy, probably with the 207 hardener. But I only vote for that over Max ACR because I haven't used Max ACR and you seemed to want the "votes" limited to actual experience on large builds. But I'll mention again that I've been very impressed with some of the advice Max ACR has given to people building plywood tanks.
Sorry for the change of vote but in one of your earlier posts I got the impression you want to build a significant composite structure on the inside of your tanks. In that case I saw little need for epoxy and I don't believe you had mentioned "cost no object" at the point.
An epoxy resin would not require a top coat unless you wanted to pigment the last layer(s) to achieve a certain look. But even then that's not really a top coat. Even if you fiberglassed inside of the tank the final coats of epoxy would act as a barrier for anything toxic in the fiberglass, fasteners or wood from harming your fish. Chemists at West Systems have discussed this with me.
Also, it was me that Buckdog mentioned earlier. Over the years I've had to discuss a few issues with West Systems and other epoxy companies. If I remember correctly, two engineers at West Systems felt it was best to fiberglass the outside of plywood tanks. A third felt at least a fiberglass "pan" should be built into the bottom to provide impact and abrasion resistance from large rocks, substrate, etc. None of them felt fiberglass on the inside of the tank had any benefit to waterproofing a plywood structure.
So if you've decided on epoxy resin I think the things you need to research would include the use of a solvent-thinned "bite coat" (some epoxy companies think they're a bad idea, others recommend them), whether to use cloth, roving or mat, and whether you want to have some fiberglass inside of the tank for impact resistance in case you're moving a large natural rock. I suppose you should also research the possible toxicity of the wetting agents in fiberglass that Buckdog mentioned.
Also look into different thickeners for making epoxy fillets along all of your interior seams. Max ACR sells a thixotropic epoxy for this. Some people just use their regular epoxy and their own thickener to make their own. West Systems told me what they thought was best to make epoxy fillets (and they did recommend them) but I just don't remember at this point. Let's just say I've had bad luck with laptops the last few years and don't have this information stored anymore.
As far as some of your other questions:
Already discussed the top coat.
Of course it can crack. But if everything is designed and executed properly I don't believe you'll have trouble with a quality epoxy resin.
Cured epoxies are much more flexible than they are given credit for. Look at all of the composite structures out there made with epoxies that are designed to flex. A epoxy composite can handle the same pressures an polyester or vinyl ester composite.
I have never put epoxy resin over a polyester composite. I have put epoxy paint over polyester composite without issue. But if this is a cost no object project I would do all of your fiberglass work with epoxy rather than polyester or vinyl ester.
If you're that much more comfortable doing your layup work with polyester or vinyl ester then you "should" be able to topcoat with an epoxy resin. It's been done before.
