Completely off in the winter (apartment building where my neighbors seem to want to live in furnaces, so the heat seeps into my apartment… sometimes it’s so hot I even have to open the windows) and as low as it goes in the summer (I hate when AC switches on and off to maintain temperature, so I set it super low so that it’s just always running… also again, old apartment building where it’s questionable whether the temperature sensor even works).

You’ve hit a very common snag with LED dimming, and your observations (flickering, hum) are spot on – it’s almost certainly a compatibility issue with your MLV transformer and leading-edge dimmer trying to talk to LEDs.

Here’s the simplified lowdown:
1. MLV Transformers & Dimming LEDs: Not Ideal.
Your current magnetic low-voltage (MLV) transformer combined with a leading-edge dimmer is causing the trouble. MLVs are great for old halogens, but LEDs are different beasts. They prefer a cleaner power supply for smooth dimming. For reliable, flicker-free dimming, you’ll almost certainly need to replace your MLV transformer.

2. The Solution: ELV Transformer + Trailing-Edge Dimmer.
The gold standard for dimming 12V AC LEDs is an electronic low-voltage (ELV) transformer paired with a trailing-edge dimmer. This combo provides a much smoother power delivery, stopping the flicker and hum. Your existing 12V AC LED bulbs should be compatible with an ELV transformer – the key is the transformer and dimmer working together.

3. Outdoor & Smart Dimming:
For outdoor use: You’ll need an IP-rated (waterproof) ELV transformer. Look for brands like Mean Well or those stocked at UK electrical wholesalers. Ensure its wattage is at least 20% more than your total LED load (your 50W needs at least 60W+).
For smart control: The easiest way to get app control is usually to install a smart trailing-edge dimmer indoors (e.g., Lutron, Fibaro, Shelly compatible with your smart home system) that controls the 230V mains supply before it gets to your new outdoor ELV transformer. True outdoor-rated smart dimmers for low-voltage systems are less common.

Key things to double-check:
Ensure your new ELV transformer and trailing-edge dimmer both have their minimum and maximum load requirements met by your 50W (10x5W) total LED wattage. This is crucial for performance and longevity.

To help further, could you confirm a couple of things?
Are your existing LED spotlights definitely 12V AC bulbs, or could they be DC? (Most garden lights are AC, but just to be sure)
What smart home system, if any, are you currently using or planning to use? (e.g., Alexa, Google Home, Apple HomeKit, Zigbee, Z-Wave) This helps narrow down smart dimmer recommendations.

Hope this helps steer you in the right direction and stop the hash!