This is going to be a great outdoor space!

You need to light it well and evenly in order to get the most use out of it. Don't use decorative lighting such as the onion light. Use architectural lighting (not about looks but about illuminating the space for good usage). Hang something like these from the rafters and exterior walls and space at appropriate intervals for even lighting. The will throw light up and down for better illumination and better beam spread than your decorative fixture.

Have them painted or powder coated to blend with the color of the wood. They come in white too.

Install dimmers to dial down the light if you feel it is infringing on wild life. Dial it up if you actually want to see.

Illuminating the old structure of this ceiling is going to give you a much better old time feel than the look of a faux old timey onion light.

I think onion lights will look right at home in this application and globes should light rafters just about right.. Some are not aware bulbs,including led are available in a wide range of temperature (color of beam immitted) because marketing hyps brightness per kw energy consumed. Lumens (quantity of light) and temperature of light are separate features that are often confused. I would choose dimmable 2700 to 3500 kelvin bulbs. If you think red,blue and other colors add to certain events, ask Northeast if they can supply fixtures with that capability. I've seen can lights which can be switched to half dozen colors including warm and cool at dimmer.

As for light pollution, I wish all my neighbors were as considerate as you are. I'm not a fan of all rules folks in HOA communities have to abide but I do wish every city would pass ordinance which says bulbs must not be visible beyond property line. To that end you might use a work light to see how far below ceiling joists bulbs can be before casting beam more than xx feet on ground leading away from your porch.

Due to wind suspended fixtures like 2012 and 2022 might not be a good option.. Ceiling mount like 2014 and 2024 can be easily adjusted up/down if mounted between joists on support spanning across top of joists. Said support can be shimmed to raise fixture, electric box shimmed to lower. Supports might be painted, wrapping with roofing copper or other treatment to blend. I would think lowest position possible would be optimal.

All enterior lighting should follow Dark Sky Principles:

I would not choose an onion light as its light will not be targeted downward to illuminate the deck; rather it will shine out into the wall or onto the yard where light isn't wanted. I would consider a row of flushmount warehouse style pendants, attached to pieces of wood strung between the two center beams (like the gizmo shown in your photo). Don't place the pendants to close to the edges of the deck to prevent them from illuminating the yard. Control the lights with a dimmer (e.g., Lutron LED+ dimmer).