@Luxie
See my replies below in different color and font.
I am looking to have the main unit connected by ethernet about 5m from our bt router. To that connect our tv and bt tv box by ethernet cables. My thinking for this is that they should be faster connections than by Wifi?
For streaming devices such as TV it is recommended to have them connected by Ethernet. This will give you more stable speeds and steady streaming quality.
Still, the formal answer: it depends. Most TVs have 100Mbps Ethernet port, as they don't need faster speeds. Their WiFi link could be faster, but it is often less reliable.
On my Sony TV I could see Netflix occasionally drops quality of picture and then quality goes up, when TV used WiFi link. That's annoying. After I wired Sony TV to Deco by Ethernet cable, I can enjoy Netflix 4K movies without such issues.
A second unit will be on same floor 10m away with one wall. To this i was going to use ethernet cables to connect PS4 and PC. This will then connect by Wifi to the main unit. Again would that be the fastest connection for those devices?
PS4 has gigabit Ethernet port and modern PCs usually come with gigabit Ethernet port. As long as this is true for your PC, connecting them by Ethernet cable will be for sure the fastest connection - and recommended one.
We will be having quite a few smart switches/bulbs and ring doorbell/floodlight. I see there is an issue with ring and deco meshs. is that the case still? Is there a work around?
The issue you mention, this is mostly because these smart devices may not understand or support WiFi mesh. For X55 specifically, there is a workaround. X55 will allow you to configure on which WiFi band (2.4GHz or 5GHz) smart device should connect and to which Deco (you'd want the nearest one to that smart device).
See the following document: How to select a preferred signal source on the Deco App
No guarantee it'll work 100% for every smart device at your place, but at least this is an option - and it is not present in some other Deco models and in some other WiFi mesh brands.
Would I be better avoiding a mesh altogether?
I am guessing you are considering WiFi mesh because your router does not provide good WiFi coverage all over your place. You should try. There is no guarantee Deco X55 set will work for you, every house and set of devices in it using WiFi is different, but for better WiFi coverage having WiFi mesh is usually a right decision. I do not regret buying my Deco mesh, for example. Haven't had family members complaining of "bad WiFi" since we've got Deco mesh.
I recommend getting Deco X55 set from the place with good and simple return policy. You won't need more than a week or two after initial setup and configuration to find if WiFi mesh works for you.
Are the x55's the best for this? (for the money) or is this a product that would be better for my needs for similar money?
For Deco brand, its model X55 is one of the most optimal in performance and features for your needs.
For example, many Deco models in that same price range have only 2 Ethernet ports on them (X55 has three). Very few will allow you to fine tune how devices should connect to them and how Deco units should connect to each other - X55 will.
One more recommendation that will simplify running Deco mesh. As you have BT router, after you set it up and did basic configuration for your Deco mesh, configure it to run in Access Point mode: How to set up Deco to work in Access Point mode
That will leave your BT router in charge of managing home network (IP address allocations and such) while Deco mesh will be providing WiFi coverage. Many benefits from doing that, but firstly: if you find Deco mesh is not for you, you can just power of Deco units, re-enable WiFi on BT Router and you are back where you started - no need to redo your home network configuration.