@Petzy Let me share some things I know about the great uniGui eco systems:
Question 1: Type of Server
Windows Server Core is a perfect platform but license cost is a big thing.
Linux Intel is possible but consider other packages, e.g FastMM4/4-VX/5.
Linux Arm is not available due to Embarcadero Delphi v11.
Question 2: Development
Yes, you can do that way. But you need a development Linux to debug on-the-go (Centos/Ubuntu/Oracle..)
Question 3: HyperServer
Yes, Linux Intel x64 is ready for HyperServer. Or you can use nginx, or cloudflare.
Question 5: Database heavy application
FireDac can handle concurrent DB clients. But you can consider using caches other streaming stuffs like Kafka to break through any bottle-neck with heavy DB, or use NoSQL (Mongodb, Firebase, DynamoDB..).
Or you can add API-tier to work with heavy DB app: prefer Java Light-4j (rest/kafka..) or Spring-boot. Due to Indy limit, Delphi API restful server is not high performance.
The total cost of ownership x time = Delphi license + UniGUI license + other libs + less-code Man-power still << opensource + full-code Man-power.
You should buy UniGUi complete license right away and do all your projects fastest!