@Rony_L
Deco S7 has three Ethernet ports. If you need three Ethernet ports on Main Deco - definitely make S7 your new Main Deco.
When you are adding S7 as Satellite Deco, Deco app may or may not offer to make it Main Deco, depending on TP-Link preferences for Deco mesh. If Deco app nags you to make S7 your new Main Deco - just agree, and if it does not - then it really does not matter, will be your choice.