Are lattice-like tables a thing already?
Yes they're a thing, if you'd done more searching you've have found something I'm sure (although I think you'd have had more luck using the word grid instead of lattice).
But generally I think you'll see something along these lines used more by power-tool woodworkers or hybrid woodworkers who are more power-tool centric. Even amongst the former they're rare, and from what little I've seen they tend not to be used as a full-on workbench but more for specific tasks such as breaking down of plywood sheets (initial cutting to rough size) with some form of solid-top bench or work table used for a lot of other processes and much of the building/assembly.
Yes, like everyone else, I place things on my workbench and wouldn't want them falling through my lattice onto the floor. But for that I figure on putting down a piece of plywood wherever convenient.
If you want to build an open-lattice top go for it, see how it works for you in practice; this is always the best approach rather than attempting to foresee all the pros and cons in your head, as inevitably there will be things (both good and bad) that you couldn't have thought of. I suggest the right approach would be to embrace it fully and simply add a shelf below and/or build in a tool well at the back. I can imagine that with one or both of these additions only a small tweak to normal working habits would be needed to get comfortable working on the new bench style, leading to putting down a piece of ply on top becoming something you find you never need to do.
Also remember that bench ends (between or on the legs/leg assemblies) can provide some useful space for added storage, at least some hangers for saws and one or more racks or magnetic strips for small tools. A simple plane till wouldn't be impossible either, although TBH I don't think such a bench would be any good for the regular user of planes.
And I know the classical solution is benchdogs and holdfasts.
That would generally be dogs or holdfasts since many older benches with dog holes had them exclusively in lines, aligned with the vice or vices, especially an end or wagon vice; we can still see the evidence of this today in the layout of dog holes in some commercial benches and many bench plans.
And similarly, a bench that used holdfasts did not have to have any kind of conventional vices because the holdfasts could do both top-clamping and face-clamping tasks, along with crotchets and various other forms of holders or stops.
But making such holes with the necessary accuracy is beyond me. And besides, trigger-style clamps are great.
Quick clamps/one-handed clamps are great. But their hold is rarely anything like what a good holdfast can provide. Plus they're way faster to set and release.
Now about those holes... they really aren't beyond you.
Because they're considered tricky or outright difficult as you can imagine many many people have problem solved the task and numerous techniques have been developed to help in the process and make them better/more consistent.
But actually a bit of variation in dog holes is no big deal. This shouldn't be a surprise since of course anyone making them back in the day was doing it by hand, and like all manual processes some minor variation is to be expected. These days with users of many different experience levels making benches with round dog holes the holes are obviously going to vary even more!
As long as the diameter is OK holdfasts work absolutely fine; they're quite tolerant of most variations except for a too-thin top. As for dogs, dogs of numerous designs can work in holes that are surprisingly imperfect — not that consistent in size, very ragged inside, torn grain at the edge (worse on the bottom1) and even if not one of them is at a perfect 90 to the top!
That said, if your OCD demands it, near-perfect holes (what you might call furniture-quality) are within the grasp of nearly anyone of reasonable skill if they're careful and go slow, using one or more tips and the right bit in the right drill2. The top edge then being cleaned up and made to look even better with a nice chamfer or roundover (generally done by router).
often, a clamp somewhere in the center would be more helpful than a clamp (or vise) at the edge of the table
Well there are ways to allow that without completely redesigning the workbench top. One simple solution is just to split the worktop in half, with the gap between left large enough to allow easy access for your clamps of preference. Benches like this are also a thing, although still rare.
But TBH it sounds like you just need to work on your conventional workholding solutions. Think of all the people who have been doing similar work to what you're doing who don't seem to be struggling using their benches with the limited access for quick clamps ^_^
And there are workholding solutions that go far beyond vices and dogs, holdfasts and a doe's foot — toothed planing stops (fixed or adjustable), flip-up or rising/falling stops, surface clamps, Wonder Dogs, poppets, wedges, cams, boomerangs, and now many new options that use T-tracks. On top of that there are sawing hooks, mitre boxes, shooting boards, and last but not least the mother of all workbench aids, the bench hook.
1 The holes in the bottom of a workbench are typically much worse on the bottom due to it being the exit hole in most cases. Breakout on exit holes with many (most!) styles of drill bit can be awful.
2 A brace and a well-sharpened traditional auger, with the lead screw/snail in good shape, can drill really great, very deep holes. Using a power drill a Forstner bit and its related cousins, or something like the Wood Owl bits can drill super-clean holes with very clean entrances; honourable mention of flat bits/spade bits, they can work far better than common perception if nicely sharpened.
