FLV Watermelon — 3% (excellent backbone; realistic yet candy-sweet watermelon profile)
CAP Double Watermelon or CAP Sweet Watermelon — 4% (adds body, sweetness, and that thick, candied depth)
TFA Watermelon or LA Watermelon — 3% (boosts juiciness and rounds it out; LA is more candied, TFA more classic)
TFA Honeydew — 2% (adds melon richness and a subtle supporting layer without overpowering)
TFA Dragonfruit or CAP Double Apple — 1-1.5% (Dragonfruit for extra juiciness/sweetness; Double Apple for soft fruit texture and "flesh" feel)
CAP Super Sweet (or TFA Sweetener) — 0.5-1% (to make it richly sweet; start low and adjust to taste)

Why this creates a "rich" profile

Multiple watermelon concentrates (FLV + CAP + TFA/LA) layer candy top notes with deeper body.
Supporting melons (honeydew) and accents (dragonfruit/apple) give it that juicy, fleshy mouthfeel instead of a flat single-note candy.
Sweetener rounds it into a dessert-like richness without being cloying.

This is a shake-and-vape friendly recipe but improves dramatically with 3-7 days of steeping (warm water bath or just let it sit). The melons and apple/dragonfruit notes meld nicely over time.
Total flavor is on the higher side for richness — if it tastes muted in your setup, increase by 1% total or drop sweeteners slightly.
Start with lower sweetener and coolant; you can always add more.