Situation  
I'm UK based. Renovated house and bought a very basic (Ce-8804) cctv system from cucctv with 4  cameras (3 bullet type and 1 small directional; the type that have BNC and power sockets) and 1tb hd. I wired this in myself.  
My house has wired cat6 Ethernet ports throughout and the cctv box sits in a main cupboard with other items in a rack-type setup 
I have a great wifi signal throughout my house.  
I've used an RF modulator to feed the split screen view out to my aerial distribution point so all TVs can tune in to view the analogue signal 
I use a Synology NAS box for personal storage and am aware this has a well-reputed camera app that lets you use 2 cameras (you have to buy a licence if you want to use more than this) 
  
Problems 
Poor video quality - Low resolution images mean if I do see anything, it's not much use for recognising anyone 
To view anything other than realtime video, I have to connect via a windows PC using their software which is terrible 
The apps they recommend using (ZMEyeFree and similar) are horrendous! 
To change some advanced settings, I have to connect a monitor directly to the cctv box and click through the options with a mouse... it's a bit old school 
1 camera is in a bad position shows more of my neighbours house and I plan to move it from side of house to rear door to be of more use 
Camera at back doesn't show to the end of the garden very well at night (garden length is approx 25m)  
system isn't smart enough to provide alerts in a form I can use, if something happens whilst I'm away (text, push alerts) 
It has developed either a hard drive or hard drive SATA cable fault so isn't recording at the moment  
Max of 4 cameras - I want 5-6 to cover my house externally (1. front garden/street, 2. rear side entrance, 3. rear right, 4. rear garden, 5. garage entrance & 6. front door visitor cam 
The cameras have audio auto its but I hear no audio on recordings or the live view  
  
Requirements/aims 
Essential  
HD (input and output) 
Clear visibility to the end of garden at night 
Cover back entrance 
Cover garage entrance hidden spot at end of rear garden  
Records audio  
Easy remote access via mobile app (preferably iOS and Android compatible) allowing realtime and specific time period access/rewind  
Allows 6 cameras to cover all external areas 
Remote device management possible  
Accessible via a MacBook Pro 
  
Hopefuls 
4K - as the next TV I buy will probably be that  
Allows more than 6 cameras - 10 would be nice to cover some internal areas 
Talkback speakers on internal cameras  
Supports IP so I can connect this to the internet of things  
Push alerts when alarm triggered  
Potential push alerts sent to specific family members 
Possible Synology box integration  
Auto/IFTTT interactivity - e.g. If alarm triggers then turn on wifi connected light in back room/turn bedroom TV on with alerting camera on screen 
Possibly including an internal cam in rear room and/or garage where break ins are most likely (no network connection currently in the garage) 
Possible integration with smart doorbell so if doorbell rang, the TVs flick up an image of whoever's outside/mobile video call  
Option to backup to Synology NAS 
Off site alternative recording storage (cloud) 
  
Budget 
Hopefully under £1000 
  
Questions  
Can I replace the current cameras with HD cams without rewiring (are existing cables suitable for a HD signal)? 
  
I gather I'll need a new cctv hub, does anyone have any experience using cameras with a synology box as their cctv pvr/security hub? 
  
Can anyone recommend a cctv system/options? 
  
Any other ideas, recommendations that more experienced installers have would be appreciated, as I've probably not considered a lot of things here.