SIBO & SebDerm

Since around the age of 12, I've suffered horrible seborrheic dermatitis on my scalp and a similar, eczema-like condition on my ankles. Topical treatments of any kind have never worked. I tried eliminating the usual suspects- gluten, dairy, and sugar- and improving my diet as well as taking supplements, but to no avail.

On occasion, I've had something that helped, but the relief was short lived.

The first time I drank half a liter of kefir, my scalp felt like it was made of actual skin for 48 hours. Never happened again.

The first time I had sardines, my condition improved considerably- presumably because of the high vitamin D content. However, subsequent results have never been as good as that first time.

Olly gut health gummies had a similar effect to sardines (probably also vit D here); provided relief the first couple times, but inconsistent results afterwards.

Only recently did I realize that many foods that I took for granted were also behind my issues. Legumes, cauliflower, apples, pears, broccoli, etc. In other words, I have a serious problem with FODMAPs.

After a bit of research, it seems FODMAP sensitivities correlate with SIBO. I found a lab that provides hydrogen + methane breath tests, but you have to pay for each sugar individually, and it costs a good bit. I don't mind paying at this point, but I'm wondering if it's worth it.

What would a SIBO test really do for me? Would it narrow anything down or just confirm what I already know? Should I instead cut straight to a low FODMAP diet?