How should I help these new plants through the heat?

For new plants in hot, dry weather, check the soil before watering rather than following a fixed daily schedule. If it feels dry a little below the surface, give the roots a thorough drink. Mulch helps slow drying, but keep it clear of the stems.

Lavender can look rough while establishing, and it is especially worth avoiding constantly wet soil. I’d hold off on feeding for now and see how it responds once the weather eases.

That much sun may be a little limited for lavender, depending on the spot, and newly planted lavender can struggle after being moved in hot, dry conditions. Keep an eye on it, but don’t try to fix every droop with more water. Too much moisture can cause trouble too.

The fern may prefer a shadier spot. If it keeps looking unhappy, moving it somewhere more sheltered could suit it better.

Have a look at the spacing before adding anything else. Hydrangeas can grow quite large and may crowd smaller plants, while different plants can want different amounts of sun and water. A few gaps are fine while the border establishes.

If some plants don’t take, that happens, especially after planting in difficult weather. Give them time to settle before making lots of changes at once.