It is best to plant spirea in early fall...earlier rather than later so your shrub has time to settle in and get firmly attached to the soil before the ground freezes.

Spirea shrubs should be grown in a spot with full sun six hours a day for the best flowering. They can, however, tolerate partial shade. Be sure your soil is well-draining. They don't need much extra watering once they have become established in your yard (usually after the first year after planting).

Lilacs can be planted in either early fall or spring. Lilacs also need 6+ hours of full sun each day for good flowering. Less sun will equal fewer blooms and also create the possibility of a fungal disease called powdery mildew. They prefer slightly acidic to alkaline, well-drained soil. They may take three to four years to establish, but once they do, they can live for many years. You do not need to fertilize them often. Doing so may result in all foliage and no flowers.

To ensure abundant flowering, cut off all faded blossoms to a set of leaves each year. Not doing this will prevent flowering for the following year. Flower buds form the summer before they bloom, so pruning branches later in the year will remove future flowers. The yearly removal of one-third of the oldest stems once the shrub is well established will prevent your plant from becoming tall and leggy.

https://extension.umn.edu/trees-and-shrubs/lilacs

As with any shrub or tree planted in the fall, be sure to keep your spirea and lilac watered until the ground freezes.

I see no reason why you should not plant additional native trees or shrubs. I just would caution you to do your homework about sycamores, which is a huge, beautiful, but not a native tree. Sycamores are best suited for soils which are moist and do not dry out. Dry soil can lead to short life for this wet-site-tolerant tree.

Here are a few of the complaints about this tree. Sycamore can be messy, dropping leaves and small twigs throughout the year, particularly in dry weather. Aggressive roots often raise and destroy nearby sidewalks. The dense shade created by the tree's canopy may interfere with the growth of lawn grasses beneath it.

The most common issue with sycamore in our area is a fungal disease called sycamore anthracnose. This disease occurs nearly every spring, but especially during a cool, wet spring. The leaves begin to crinkle, then turn brown or black and fall. Some trees are nearly totally defoliated; however, if the tree is planted in an appropriate site and is well cared for, most years it will flush out a new set of leaves.

This can be a messy tree. If the tree does develop anthracnose, it is important that the fallen leaves be raked up and destroyed. The fungal spores are moved to other trees by wind and rain. These spores can still viable and re-infect the tree the following year if not removed. Trees that are repeatedly defoliated could be reduced in vigor and be more susceptible to other problems, so steps to promote good tree health can be used as a preventative measure.

Here is some information about this disease:

https://www.missouribotanicalgarden.org/gardens-gardening/your-garden/help-for-the-home-gardener/advice-tips-resources/insects-pests-and-problems/diseases/fungal-spots/sycamore-anthracnose

Sycamore is a very beautiful tree, and sometimes none of these problems occur. I am not trying to discourage you from planting a sycamore; but if you decide to do so, look for one of the newer varieties that may be more resilient (not immune) from anthracnose.

Thank you for your kind words, Penny. I too live amidst mature oaks, maples and white pines. “Pick up Sticks” was a fun game as a child; not so much here in the woods. Good luck with whatever you plant, and most of all…enjoy the process!