While I can't speak to your specific situation, I can offer my own fusion experience in hopes that's helpful. I had C4-C6 fusion back in 2016 and am in better shape now than I was before the fusion.
Yours is a lot lower down, and I understand that lumbar fusions are a little more limiting than cervical.... but the precautions are probably pretty similar I'd imagine.
My Dr told me same thing as you, no weights at all for a minimum of six months and to take it easy with anything I did do.
I wasn't able to sit idle for 6 months, so about two weeks post surgery I was in the gym 3-5 days a week just walking on the incline treadmill. I lost some motor function post surgery, so I had a hard time walking - we're talking 1mph at 5° for 30 min or so when I first started. But I was consistent about it, and pretty soon was doing low impact walking at 2.5-3.0mph on a 13-15° incline. Important to keep it low impact after a fusion surgery since it destabilizes the levels above/below, so unless you want to have another one in 5-10 years I'd avoid jogging/running/mountain biking.
Bout three months into my recovery I'd gotten to the point where I wasn't in pain all the time, didn't need to wear my brace, so I started adding in weights here and there. Really light weights, I'm talkin sets of 30-50 reps. I was surprised at what I was able to do, and what I wasn't. Like, you'll FEEL it if you put any stress on your healing spine - so I found all the exercises that I couldn't feel in my fusion area and did those. For example, I couldn't do standing curls or cable tricep work - felt it in my neck hard, but I could do leg extensions, single leg presses on a machine, body weight squats, dumbell flies, stuff like that. My workouts were all over the place for a while. Most compound lifts are out of the picture because it's hard to find one that doesn't stress your whole body, so you're not going to be doing any deadlifts, barbell squats, overhead presses or anything like that. Just look for anything that you can isolate to the point of not feeling any sort of stress on your fusion area when you perform the exercise.
I did that for about 3 months, then I had my 6 month follow up with the doctor and my fusion was deemed to be full and successful and they said I could start getting back into weights. Because I'd been doing light work for quite a while already, once I started adding in weight again it really didn't take me long to get back where I'd been before, and I kept up the walking cardio because I'd gotten used to to doing it and liked taking an hour on the treadmill to listen to podcasts or watch the news or what have you.
For bone density, did they send you home with a electrical bone growth stimulator? Those things are one of the better things you can do to accelerate your healing, but you have to wear it literally every minute that you can to get the full benefit. If you smoke - quit, that's one of the worst things for healing from spinal fusion. Protein intake is important, as you can't build bone without protein. Vitamin C helps with bone healing through it's antioxidant effects. I was told that Vitamin E was good to take too, so I took it daily for my recovery period - but I can't say whether or not it was effective.
Gear isn't something I know about really, so I'm not sure if there are any benefits to taking stuff like GH or Test to bone growth. I'm sure someone on here can opine on what options there are in that realm of pharmacology.
