How long are you looking to go and what is your access to water like?

If I need more than 3 bottles I typically bring a water filter or plan to make stops at stores.

I have this one:
MSR Trailshot

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300 miles on rail trails and gravel roads etc in one day, should be stores etc, where we can refill. plan is to stop briefly for water etc every 50 miles

Another option is to do what the endurance mountain bikers do: wear a hydration pack like a Camelbak. If I'm doing a super long deal, I'll keep sweet stuff in my bottles, and keep the Camelbak with pure water. Also good for carrying extra powder, food, etc.

Edit: though 50 mile chunks on "fast gravel" should be quick enough that you might not need to go full pack. Depends on temperature, etc.

One tip I've seen for bottles down there is that when you go to use it, switch caps, ie put the clean cap from the bottle inside the triangle on it.
Obviously water is heavy, so you want to carry the bare minimum, ideally taking the very last swig as you pull into the refill spot haha. I've used one or two of these Moosetreks handlebar/stem bottle holders, and they're actually really nice. After using one, I start thinking that having to reach down in the triangle is a big hassle :) Can put a bottle in one and snacks in the other.
I know a lot of people don't like them, but personally I don't mind wearing something like a Camelbak Chase vest. A 50oz vest is pretty small in the grand scheme, but still is enough to replace about 2 bottles. More hassle to refill, though.

Personally, i will not put anything that i intend to eat or drink down there... too many road debris.

You may as well consider drinking from an unfiltered creek...

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Alternate ideas: use bottom down tube mount for storing your powder in a zip-lock, to prevent contamination from road debris. Or put spares/tools down there in a ‘’keg” canister or bottle, so you can run another bottle off the saddle. Or get a 1/2-size frame bag and put a 2-3l bladder in it, with a small bottle still able to fit in the triangle (hopefully). Camelbaks are also a decent option, especially if you want space to carry extra calories.

That lower cage is nearly impossible to get to while in motion unless you have very long arms. If your frame is big enough you can run two cages/bottles on the downtube using a WolfTooth b-rad and side load cages.

Another thing you can do at each stop is not only refill your bottles, but also your body. When I go for long rides I drink a few pints of water before I leave and I need to carry less water. It's a tactic ultralight hikers use to carry less.

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Strava

A few companies make either 950ml or 1000ml bottles. I like the ones from Elite.

Your normal bottles are probably between 20-25ounces. So, having 2 bottles that are around 32oz each gives you almost the equivalent of 3 normal bottles.

Putting a bottle of powder below your down tube isn't a bad idea either. You could also look at a handlebar bag to store a ziplock bag (or multiple single serving bags) of powder. Not aero, but it's easy to get at and you could store other food in their too.

The only way to tell is to fit a cage and go for a ride. I recently bought a used dual-suspension bike that I turned into my main gravel/all-road bike and it had a downtube and and under-downtube boss. I figured I would put a cage down there and then switch them out when the easy to reach one was empty. Then I found that it was not that hard to grab and just use it normally. It does get a little dirty, but I never found it to be that big of a deal for me.

You won't ever be drinking directly from that bottom mount on the fly. Maybe you will use it to transfer to another bottle or only drink from it at a stop.If you decide to keep something on that bottom mount, the best I have found (for keeping the nozzle clean) is this Nalgene bottle.

Here's the link for all options.

That bottom mount is a good place for tools/spare kit too.

After putting it off for a few years, I'm pretty pumped about frame bags now. The Revelate Designs Tangle frame bag in large fits my 58 frame perfectly with room for two bottles inside of the frame with a Wolf Tooth B-Rad 2 slot dropping the seat tube bottle down a little bit. There's lots of cool bags out there, but this was everything I could have wanted for $90. For super long days, I really like not having stuff on my body. I'll be a little wasteful and use that for my 3rd bottle and buy something from a gas station every couple hours whenever we stop. I can't drink the exact same mix 6 hours into a ride, so maybe I'll toss a tea, couple sodas, Gatorade or something random in the frame bag to look forward too. It sits inside of the frame without sticking out of the sides too much, so I would like to believe it's still somewhat optimized for speed.

I'd run two bottles in the normal spots then two under the saddle (TT / Tri style). https://amzn.to/3jNh3DA

I'd also run a half frame bag for stuff. You'll probably need a side-exit bottle cage - Elite Cannibal XC are cheap and good. https://amzn.to/3077sjd

Move your flat kit from the saddle to the down tube https://amzn.to/2DggUaW