Actually, 5-HTP mostly does get to the brain. Tryptophan, on the other hand, usually doesn't. Most serotonin isn't in the blood, it's in the digestive system. I don't think you'll find a bunch of heart valve patients from taking 5-HTP. It also doesn't necessarily increase serotonin beyond normal levels -- if the body doesn't need it, it won't utilize it and will break it down. Nothing is risk-free -- drink too much water and you'll drown. But 5-HTP is certainly a lot safer than any pharmaceutical product. If you want no risk, well, being alive isn't going to be like that.
http://www.progressivehealth.com/5-htp-heart-disease.htm
not take 5 htp and other natural serotonin increase substances as using then for long time can cause heart valve problems as only a small percentage of serotonin gets into brain while rest circulates in blood and start to affect heart valves.
5-HTP is an amino acid, as such is most likely best taken with a meal. It is generally dosed two or three times a day depending on the dosage to keep it in your system. Taking it before bedtime won't do anything to help it work, as it needs to be in your system on a regular basis if it's going to work. I believe the dosage listed for SamE are probably too high as well. Be aware that some people get anxious on these two substances, as they can on antidepressants -- SamE particularly can be stimulating, as it's involved in the energy production system of the body. St. John's Wort has also proved useful for many people, but don't take it if you have photosensitivity. But none of these will "cure" you any more than antidepressants will cure you -- they help with symptoms. The "head-shrinking" you dislike is the only known cure to mental illness that can go beyond just mitigating symptoms, but there's no guarantee it will work, so there's that. But if it does, you're done. So I would try it despite your misgivings. If you really don't have a good knowledge of natural medicine, I'd see someone who does to guide you -- usually remedies are combined to work on different parts of the body for best results, but therapy and lifestyle changes are always included in the natural treatment of mental illness. A good book for an overview of natural remedies is Natural Highs by Hyla Cass, a psychiatrist at UCLA. Good luck.
There are a couple natural supplements you can try for depression. The best I think is SamE. Be sure it is enteric coated, and start with 200mg, increasing every couple days by 200 mg until you feel a mental lift. No more than 1,200 mg per day. Take it first thing in the morning on an empty stomach.
Poor sleep can contribute to depression...Be sure you are getting good sleep, which is hard for a truck driver, I would imagine. You can try 5-HTP up to 300 mg taken about 30 min before you go to bed, and melatonin is good for sleep as well.
Also be aware that depression is a common side effect of many blood pressure meds. You should talk to your doctor about it and maybe he can switch you to something else that won't cause depression.