I have trying to do this for a while and recently came across something that really worked well.
You need Biogaia L Reuteri probiotics (
https://biogaia.co.uk/collections/gut-health-for-adults/products/gastrus)
The recipe is as follows.
The first time:
Crush up a tablet of Biogaia into a powder
You need about 8 teaspoons of Dextrose per Litre of Milk
A litre of milk
Mix all of that well and put into a yoghurt maker(these can be about £20
https://www.amazon.co.uk/Stainless-Adjustable-Thermostat-Homemade-Desserts/dp/B0B2Q7LB9S/ref=sr_1_3?keywords=homcom+yogurt+maker&qid=1692280373&sprefix=Homcom+yo,aps,77&sr=8-3 if you can find one), make sure you have sanitised it with boiling water.
Make yoghurt at 37C for 36 hours, put in the fridge and then it should be ready to eat after about 4 hours of cooling and should give you a gooey yoghurt but not firm/jelly like nor should it split.
You can use a 8 teaspoons of the previous yoghurt to make the next batch with milk and dextrose. Whole milk works better but its the dextrose they eat so apparently coconut milk also works.
I wont go over what didn't work as I have been trying to make this type of yoghurt decent for 2 years. This is the recipe, dextrose is the magic answer.
I have also confirmed with a biomesight that this has resulted in a permenant change to my biome, L Reuteri now turns up in results without taking in the yoghurt for weeks before hand.