No worries. Everyone starts somewhere. If you want to solve the TrĂĄdfri bulb issue, one of the repeaters paired to the Hue Bridge might solve the problem. Pair the other three repeaters with HE. Place them strategically in your concrete fortress. I've heard how Australian homes block all signals by nature. Pair the Xiaomi temp sensors and ST door sensors with HE as well.
The Hue and HE are operating independent Zigbee networks, and that's good. Keeps the lights, which typically DO repeat just beautifully amongst themselves, but often do not want to consistently repeat Zigbee signals when other devices are mixed in, on their own and happy. However, there are some people that have certain combinations, and say it works. Best and easiest, in my opinion is to just separate them. It's possible to do it well, and it works reliably. If you haven't installed it yet, you'll want to be using the Hubitat Hue Bridge Integration app.
I have a Xiaomi Aqara temp sensor. It works fantastic, but I do have a TrĂĄdfri outlet acting as a repeater 3 feet from it, so that needs to be considered. I have Xiaomi door sensors, and they work great with the TrĂĄdfri outlet downstairs, which is only 3 meters from the hub. I now have one additional repeater, also close to the hub. But, I don't have the barriers that you do with the house construction down there. The ST 2018 sensor I tried worked well, but I don't own any ST contact sensors, so I don't have personal experience. I've heard nothing bad about them with HE.
In regard to general health of the Zigbee network, you'll want the hub as central as possible. That can be unfeasible if the construction simply doesn't allow it. @Cobra in the UK uses a long USB extension to get the Z-Wave and Zigbee dongles just where he needs them. Several other users do the same, and perhaps you are doing that already.
The IKEA TrĂĄdfri repeaters you bought are these?
They will pair with HE as device. This is correct. You simply use the USB port on them if you want, but they are just devices on the Zigbee network, helping to relay the signal between the devices and the hub.