Bare unbranded extruded aluminum fixtures can be had COTS and cheap, and you can either get a board house to design boards for you, or link up starboards to populate the fixtures. A small SMT oven can be had for starting at $500 out of China, though making an SMT oven to solder components to a metal PCB is as simple and cheap as a couple of half-silvered halogen bulbs in a workshop fixture, and a reflective metal top replacing the cage. Due to the nature of metal-PCB rework and how it basically ignores thermal profile recommendations, don't worry too much about following a specific thermal reflow profile on the first reflow attempt, just make sure you're getting hot enough at a decent-but-not-blistering speed to ensure all solders down, and then back off the heat immediately and allow the board to cool down. Adjust the height of the board from the light fixture or adjust exposure time or both as necessary to deal with soldering issues.
Plastic stencils that match your LED/component pad profile/layout are easy and cheap to manufacture (or buy pre-made.) Solder paste is fairly cheap (though there are some that get expensive, like the indium-bearing solders.) Most components are relatively simple to hand-place as long as your solder stencil job is properly-done and the components aren't too tightly-packed together and tiny, though small high-speed dual-nozzle pick and place SMT machines can be had for around $5K ($2500 or so used.)
I suggest looking into servicing the UV market, as that is the market that is blooming. The pandemic has boosted the need for disinfection-capable UV LEDs (specifically UVC,) and also this becomes a boon for other markets, like fluorescent mineral photography, minerals location (things like scheelite are easier to find via their glow when excited by UV radiation at certain wavelengths.) UVA is also showing some experimental results at disinfection, and despite the need for more power to achieve the results, UVA LEDs are much more efficient than UVC, and much cheaper to obtain, so there might be potential there as well if more research is performed and validated.
Alternatively, you can go the simpler route of being an assembly house instead of a manufacturing house, buy fixtures, and pre-populated boards that are designed to mount in said fixtures, make working units, sell those. I did precisely this with another person, literally out of their garage, making aquarium lighting for a while - LED strips, screws, extruded aluminum heatsinks, plastic 'lens' inserts, power wire with switch, water-sealing grommets, power supplies, extrusion end caps. If you work hard, make the right development decisions, and advertise effectively, you can easily grow into something that is sustainable and rewarding.