Hello, @jocellin.lee! Thank you for helping to connect the community! 
If you just run your own node, without registering as a pool - then you just spend money on the server support, without making any more profit, than you would make, delegating to some existing pool. The math goes something like this:
If you delegate your stake to the pool - your profit is Total Block Reward minus Total Pool Fee. But when you run your own node, without beeing a pool - your profit is Total Block Reward minus Total Server Cost. In this case if Total Server Cost is less then Total Pool Fee - then it is more profitable to run your own node, but the probability is very high that Total Pool Fee will be much lower than a Total Server Cost would be, at least for many pools.
But, if you not only run a node, but also register as a staking pool - then you could also set up your own pool fee, that would help you to lower the Total Server Cost for your node. In fact, I would argue that - the whole point of staking pools is so that delegates could โhelpโ node-owners to lower the cost of maintaining a node in return for profits
The whole system is optimised toward many small pools.
You can use ADA as a payment service right now. When you launch the Daedalus - thereโs a full-node automatically starting in the background. And this full node has itโs own API -
https://cardanodocs.com/technical/wallet/api/v1/, So you can just use it to create your own application that will check transactions from your clients. This is how exchanges work.
Smart-contract development (like Ethereum) will be possible toward the end of the year, maybe at the beginning of the next one.