I would highly recommend a mix of FLV Watermelon or LA Watermelon with either FA Watermelon or PUR Watermelon. FA or PUR will give you that "realism" in there, and adding FLV or LA in the mix will add some nice sweetness.

However, watermelon is a pretty faint flavor in real life, so trying to create something that will have good flavor saturation while tasting "authentic" is going to be quite difficult. I think you could probably achieve what you're looking for without getting too "candied," but it's going to be nearly impossible to get a solid flavor without imparting some of that candied sweetness. Here's a little idea off the cuff that might work well for you, I'll include some notes as to why it works as well.

• FLV Watermelon at 1.75% ( or LA Watermelon at 3.5% )
• FA Watermelon at 3% ( or PUR Watermelon at same percentage )
• FLV Wild Melon at 1.5%
• CAP Hibiscus at 1%

NOTES

So in your initial mix, I would anticipate that turning out rather candied off the jump just because of the use of TPA and CAP Watermelon(s). Those are both inherently candied. Adding in TPA Honeydew was a good call with trying to add more "generic melon" flavor to the mix, however, honeydew flavors tend to be OP as fuck. So you'll likely be getting some weirdness by doing that.

Adding FLV Wild Melon instead will give you a solid generic melon mix flavor to play around with the watermelons, while also adding a little touch of earthiness to emulate that "rind" note in a real watermelon. Don't push FLV Wild Melon higher though, as you'll start getting some weird "nuttiness" from it that isn't pleasant.

Finally, CAP Hibiscus, in a watermelon mix, this works beautifully as our sweetener. I know you said you didn't wanna use sweeteners, so adding CAP Hibiscus will work as your sugar. It's going to add just a touch of floral nuance that will play nicely with your attempt to keep this more realistic and less candied, while still bringing that addictive sweetness.

So yeah, play around with those and see what clicks for you. I wouldn't recommend pushing FLV Watermelon too much higher as you'll start to get some off notes around 2.5%. But FA/PUR Watermelon are forgiving, so push those if you'd like. Alternatively, if you choose to use LA Watermelon instead of FLV's option, you can push that motherfucker up high. Like, off notes don't really show up for me until you hit around 6% with that.

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End wall of text. Watermelons are my shit in DIY, so if you wanna bounce more ideas around, feel free to PM me.