UPDATE: I fitted new tubeless tyres today using the Muc Off sealant. It is definitely much thicker than the Stans sealant I used last, but I managed to inject sealant using a syringe via the valve stem (valve removed).

I use this - https://komcycling.com/products/tube...e-removal-tool

The sealant also dried up / sealed up quickly, and "plugged" the syringe nozzle between me filling one tyre and getting around to the second tyre. That's good going I think, technically doing it's job. I'll update again if my valves get clogged up or I have any issues.