Based on your answers, your follicles are likely being miniaturised by DHT while losing their oestrogen protection. That is a root cause, and it is one of the few that is actually addressable.
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The Pair That Works Better Together
The softgels or drops handle the hormone behind the thinning from the inside, and the serum goes on your scalp to wake up the follicles that went quiet. One slows what you are losing while the other pushes what is coming in, which is why they do more together than either one alone.
The Growth Duo
You already tried biotin and thickening shampoos. Here is the gap they left: biotin feeds the hair shaft but cannot stop the hormone shrinking the follicle. Shampoos work at the surface. The root issue is internal hormone signalling. Neither addressed DHT.