Sure, you can stack those sats and sell them in the future when they're worth more, but then you could also just buy small portions of BTC every day.
That is true. How much Bitcoin could I earn by directly purchasing $15 per month of BTC; or how much BTC would collect in a month with mining having the same cost.
Purchasing BTC directly is usually going to be cheaper than mining with most micro-miners and residential electricity.
About 4TH would give you how much BTC currently, for an average month? So a few sums up, you get around 0.00024465 BTC / 9.62 USD. Less than buying directly.
Exactly; that's true (even more true) with comparable devices, though. As shown in the efficiency chart from my original post, only the last few generations of industrial miners have a lower energy per hash ratio, i.e. higher hashrate per unit of power. And again, residential electricity prices suck, especially in Europe.