Multiple subnets, hotspots and OSPF routing on a relatively small network

RouterOS Wireless Networking
1

I would like to show the following example for a setup on a research station where I work, as we have a low budget, I have done a MTNCA course and have to solve network issues by myself and have been going through a steep learning curve over the past 2 years. The network has at any given time about 40 users, I have a high load of data transferred over the LAN, and at night I synchronise to the near max of our satellite upload to backup data to servers overseas.
I would like your opinion on the current setup that I would like to put in place to upgrade the network, as currently the network is very inefficient and users have a high latency over the LAN (to either of my servers) or to the Satellite connection to the internet.

Please let me know where I should explain more or I should put more details about the configuration or exports of the multiple mikrotik devices.
As per image below is the setup I would like to install, I have the devices mostly in place and am running some tests already. However I am running into the following issues and am wondering whether I am taking the right route.

    1. DHCP: should I have a DHCP at each site, or central DHCP at the main mikrotik


    1. Hotspots with Userman: Should I have a hotspot at each site with a radius / userman server at the Mainmikrotik?


    1. For QOS local traffic and WAN traffic, what would be the best QOS protocol to use?/


    1. General topology: would it be better to use one subnet, if so, how would one direct the OSPF routing to ie. the server at the Farmhouse etc.


    1. Mixed brand usage: As I have no other choice than working with what I have, I’ve got the ubiquity nano and airgrid for PtP connections, what would be the most efficient here to replace if any of these devices would need replacing?