@Jr3al
Welcome to a fascinating hobby. You clearly have more horticultural experience than many who are "New to Bonsai" that come here looking for help.
It is really difficult to teach a total horticultural novice everything they need to know to grow trees indoors, under lights in the format of a forum, where a post more than 200 words is just "too long" to be worth reading. Sound bite horticulture. So for many that enjoy posting answers to questions on this forum, it is just easiest to say "you can't raise junipers indoors". Because that is less typing. But when all these posters are pushed, most, if not all of the know of, or have seen trees belonging to a "bonsai famous" bonsai cogger, Jack Wilke. Jack is currently in his 80's years, and has been raising junipers and Chinese elms under lights for 365 days a year, for over 40 years.
Attached is Jack's 2002 article he published on his trees, and at that time, state of the art under lights growing was 40 watt fluorescent shop lights. Not even "marijuana grade" lights. He would not have been able to get veg growth of 10 inches tall under those lights. Yet some of his junipers, cupressus species, chinese elms are currently still alive, approaching 40+ years old under lights. And he is still alive and living in Michigan, there are those on this forum who have his phone number and could reach out to him. But majority of the younger members of this forum keep forgetting Jack is out there proving you can grow Junipers under lights, all year round. So ignore the nay sayers. You answered the vital question, growing outdoors is not feasible for you at this time. When you post in the future, if you add that sentence, "growing outdoors is not feasible at this time", that should be enough to stop the haranguing about "you got to put it outside".
So lets talk about growing indoors. One element of indoor growing is there is a limit to the amount of cool or cold that can be provided in winter. So clearly only sub-tropical and fully tropical species are really appropriate for indoor growing. Fortunately this is a long list. Chinese elms and procumbens junipers happen to have native ranges that extend into sub-tropical regions. They can cope. Both are high light species, but fortunately marijuana growing industry has made finding bright enough lights relatively easy and almost affordable. Better species for growing under lights are the tropical and sub-tropical species that are listed as growing in shade, and or part shade. Full sun tropicals are often tricky to keep happy. An obvious candidate for under lights is Ficus, the tropical fig trees. In particular Ficus microphylla, which includes the tiger bark and Green Island Ficus. Also the willow leaf Ficus, and Ficus benjamina are all good ones for under lights.
Flowering trees for under lights, Malpighia - Barbados cherry, and Eugenia, the tropical bush cherries, are good for under lights. If you have a cool room in your house where in winter temperatures can be kept near or slightly below 60 F or 16 C for 6 weeks or so in winter, you can grow "florists azalea", these are the azalea that were bred for easy bloom forcing. You find them in the floral and gift department of big box stores, not in the landscape department. These work nicely under lights, because they were bred for easy blooming in a greenhouse set up. Gardenia and Citrus also have lovely fragrant flowers, and do moderately well under lights. Carissa the South African shrub called Cape gardenia, does well under lights, note Carissa is not related to Gardenia, even though they share a common name.
So the trick with under lights growing is to deliver enough light, either by "brute force" meaning high intensity, or by the trick of compensating for lower light intensities by using longer day length. Without going into details the "standard practice" is 18 hours of lights on, 6 hours lights off for maximum photo efficiency. Day lengths over 18 hours have been shown to be a waste of electricity, with 24 hour lights on being detrimental to the plants. Anywhere from 12 hours to 18 hours lights on period will work. If plants look like they are weak from lack of light at 12 hours,, just keep increasing day length until they look good. Stop increasing at 18 hours.
So you now have more than 250 words from me, hope you make it to the end of this post. - Leo
