There is no particular reason to embrace streaming if you are happy enough with your current set up. In fact it might be a negative being on a fixed income, between the cost of new gear and the monthly cost of a streaming service.
Essentially streaming is a different way of hearing digital music. You can either hear digital music from your CDs, or you can copy those CDs to a hard drive. A streaming service keeps those recordings on their hard drive somewhere else, and you pick the ones to listen to with a device such as a tablet or phone, and you pay a monthly fee.
Think of the brand names you mentioned as being analogous to the brands of cars. Streamers are essentially computers, gussied up to look and sound like audio components. They are subject to the same problems that networking computers are prone to.
DAC is Digital Audio Converter. It converts the 1s and Os in the stream into sound. CD players essentially have two parts: 1) A transport, which reads the the data on the disc, and then sends it to 2) the DAC, which converts the digits into sound, and then sends it to the amplifier. Streamers also have to send digits to a DAC to make music. Some come with DACs included, some without.
In your situation, why would you want to explore streaming? First, the large services offer a lot more choice (albums) than most of us can collect. Second, streaming makes it easier to pull in Internet Radio, and you might find that very rewarding. Streaming also makes it easier to obtain Podcasts
There is a constant debate as to what sounds better, streaming or CDs. At the risk of enraging many here let me editorialize that the difference is minor, equipment dependent, and possibly not apparent to aging ears.
I can’t help you with the RTR question. Try asking in the Analog Forum