One question please: how many mesh nodes are there in total on your network?
You are correct about the 4MB Flash limitation with recent versions of OpenWrt.
As you are no doubt aware, the OpenWrt project was forked in 2016 to form the LEDE project after OpenWrt 15.05 (CC) stable release.
The LEDE project produced the LEDE17.01 stable release in 2017.
The two projects then merged and now have produced the OpenWrt 18.06 stable release.
SECN 4 firmware is based on OpenWrt 15.05 (CC) stable release which supports the 4MB Flash devices like the MR3020.
SECN 5 GA1.0 firmware is based on LEDE 17.01 firmware which also supports 4MB Flash devices. However, there is not a lot of room left in the Flash memory space to load packages, so there are some limitations on what facilities can be included.
SECN 5 GA1.1 firmware is based on OpenWrt 18.06 which does not support 4MB Flash devices (in general), and specifically not the MR3020.
In practical terms, MR3020 and similar devices are now limited to running OpenWrt 15.05, or perhaps LEDE 17.01, but ongoing support and updates for these will obviously be limited.
Conversely, newer devices such as the AR300M and AR750 are supported in OpenWrt 18.06 onwards.
Older devices with 8MB Flash (such as the WR842 V1, V2 and V3) are also supported on OpenWrt 18.06, and so are supported on SECN 5.
In the SECN 5 firmware, we made a decision to switch from using AdHoc to Meshpoint wifi interface for the mesh, because ongoing development is focused on Meshpoint, and not AdHoc.
This means that you can't mix SECN 4 and SECN 5 devices on the same mesh.
So if you want to move to SECN 5 to utilise the newer devices, then you have to leave the older 4MB devices behind.
(One caveat - technically you can switch from AdHoc to Meshpoint in SECN 4 by making some changes in the code, which would allow you to operate SECN 4 devices on a mesh with SECN 5 devices. This could extend the life of your MR3020 devices, but they would eventually run out of support on OpenWrt 15.05)
To address your specific options for upgrading your network:
Option 1 - Switch to SECN 5 and replace all devices with newer devices (operating on 2.4GHz).
This is certainly a viable option.
(Personally, my preference is for the AR300M device over the MT300A device because the wifi drivers for the Atheros chipsets seem to be more mature that those for the MT chipsets.)
Option 2 - Use a mix of existing WR842 V3 and some newer devices all running SECN 5.
This is also a viable option as the WR842 is supported in OpenWrt 18.06 and SECN 5
(Caveat - I test WR842 code on V1 devices only as that is all I have. But others have tested V3 devices with SECN firmware previously)
Option 3 - Use AR750 devices to replace all existing devices
(I presume that in this option you would use the 5GHz radio for the mesh and the 2.4GHz radio for AP.)
This is also a viable option, and gives you the advantage of being able to run the Mesh and APs on independent radios in each device instead of having to share the resources of one radio across both functions.
You would have to test the 5GHz radio in your situation to ensure that it will provide good connectivity around the buildings, given the difference in propagation characteristics between 5GHz and 2.4GHz. The AR750 has internal pcb antennas for the 2.4GHz radio, but has a separate 5GHz antenna mounted internally and connected via a standard pigtail connector on the board. This would allow you in theory to fit an external antenna to improve the signal propagation in your situation (keeping in mind the legal limits on effective radiated power in your jurisdiction).
Of course you can also use the AR750 to provide a 5GHz AP which might be useful in situations where the 2.4GHz band is overcrowded.
I hope this helps. Please don't hesitate to ask further questions if necessary.