Help me spec out a replacement home network using Ubiquiti bits

Build-Troys-Network

That's it - I've finally lost it with Linksys and both my WRT 1900ACs that are only a year old are getting chucked. Don't get me started on all the reasons why, but it's primarily down to continued degradation of wifi signal and the constant need for reboots. Going by the responses to this tweet, that's just what they do: https://twitter.com/troyhunt/status/778867707655487488

I’m going all out with Ubiquiti instead. No, I'm not interested in [insert the other thing you think rocks here], there's a really vocal majority in favour of Ubiquiti so that's that. Now I need help speccing out what I need for my house as it’s not quite as straight forward as just chucking in a couple of (dodgy) routers.

Here’s what I’m working with:
- Large multi-level house about 500m2 (needs at least 2 APs, probably more)
- Wired ethernet to every room (I believe Cat 5e, was here when I got here)
- Patch board in the garage and a 100Mbps hub (running patch cables out to a Linksys 8 port gigabit switch instead)
- 4 wired connections used in the lounge (presently has 1 Linksys WRT 1900AC + ISP cable modem which needs to be the because that’s where the cable enters)
- 6 wired connections used in office (presently had 1 Linksys WRT 1900AC + a Linksys 8 port gigabit switch)
- Internet connectivity: Cable modem -> Linksys WRT 1900AC (lounge) -> patch board -> Linksys WRT 1900AC (office)

Here’s the Ubiquiti bits I think I need:
- UniFi Security Gateway (sits between switch and cable modem): https://www.ubnt.com/unifi-routing/usg/
- 3 x UniFi US‑8‑150W 8 port UniFi switches, 1 for the lounge, 1 for patch board and 1 for office, both with PoE: https://www.ubnt.com/unifi-switching/unifi-switch-8-150w/
- 2 x UAP-AC-PRO access points for lounge and office (or go all out and get a 5 pack of them): https://www.ubnt.com/unifi/unifi-ap-ac-pro/
- UniFy Cloud Key to manage it all: https://www.ubnt.com/unifi/unifi-cloud-key/

Network topology wise, it then does this:

           Cable modem in bridge mode (lounge)
                            |
                            |
             UniFi Security Gateway (lounge)
                            |
                            |               Wired lounge devices
                            |             /
              US‑8‑150W switch (lounge)
                            |             \
       UAP-AC-PRO           |               UAP-AC-PRO for lounge wifi
    (somewhere else)        |
                     \      |
                       US‑8‑150W switch
                     /   (garage)
       UAP-AC-PRO           |
    (somewhere else)        |       Wired office devices
                            |     /
                       US‑8‑150W switch
                        (office)  \
                                    UAP-AC-PRO for office wifi

Questions:
- What would you do differently / better?

Thank you!

@basisbit - I decided to update the firmware tonight (1st time in 6 months) and now both AP's are now connecting at 1Gbps. Go figure!