The moisture pattern
For Theraphosa blondi, use moisture-retentive substrate with very good ventilation; never swampy and never allowed to dry completely. Use that as a practical pattern across the enclosure instead of trying to make every part of the substrate equally wet.
Watch how quickly the Goliath Birdeater enclosure changes over several days. The same amount of water behaves differently in a sling tub and a larger adult setup.
Water dish
Keep a clean water dish available to Theraphosa blondi once the spider is large enough to use one safely. Refill it before it stays dry for long periods.
Fresh drinking water and substrate moisture serve different purposes in a Goliath Birdeater enclosure; one should not be used as an excuse to ignore the other.
When the enclosure is too wet
Persistent condensation, a stale smell, widespread mould and substrate that never begins to dry are signs that the Theraphosa blondi enclosure may be staying too wet.
If that happens with a Goliath Birdeater, reduce how much water is added and improve airflow where necessary. Do not solve excess moisture by removing the water dish.
When the enclosure is too dry
If the Theraphosa blondi substrate dries through very quickly and the species needs a moisture reserve, add water to a controlled area or use more appropriate substrate depth.
Small Goliath Birdeaters can dry faster than adults, so juvenile enclosures often need more frequent checking even when the adult species is not kept damp.
Seasonal changes
UK heating in winter and warm summer rooms can change how quickly the Theraphosa blondi enclosure dries. Adjust watering to the enclosure itself, not to a calendar.
The care target for a Goliath Birdeater stays the same through the year; what changes is how quickly the room and ventilation move the setup toward drier conditions.