Single domain for all niches Vs Separate Domain for each niche

First off, I would think the alexsmith.com domain ship has long since sailed. But even if you went with a different nom de net, you'd need a hefty budget and/or a lot of patience before you begin to earn any worthwhile brand recognition. And being known by a brand name in one area won't necessarily help you to get traction in another unrelated field.

The idea of alexsmith.com/weightloss and then moving on to alexsmith.com/dating (or whatever main name you use) is a non starter, in my opinion. Apart from the issue of transferring to different fields mentioned above, categories like dating, weightloss and self-help are huge markets comprising hundreds of niches and sub-niches. You'd be unlikely to even scratch the surface with that strategy. Same thing for the weightloss/alex.com idea.

If you're just starting out, the better course of action would be to find a much smaller niche within one of the markets you've mentioned. For example, in the weightloss market there are niches catering for pregnant women, seniors, busy executives, work-at-homers, soon-to-be-brides etc. Then there are programs covering vegetarians, vegans, paleo, low carb diets etc., not to mention the various weightloss-by-exercise sub-niches, like kettlebells.

Concentrating your efforts on a smaller and tightly-focused segment of a profitable market would make it easier to target your prospects and become known and trusted in your area of specialization.