Acidity with Heritage

I need some advice. I am in the UK.
I have been happily been making strained yoghurt from Yeo Natural Yoghurt or FAGE 10% as a starter.
My milk is UHT 4% fat, so doesn’t need pre-heat.
I sterilise with boiling water from a boiling water tap and then make in a Lakeland maker (single 1.5 litre container) at 38° C for 16 hours.
Strain it and it’s mild like my spouse likes.
However I feel I need to start again every 3 or 4 batches since the taste drifts.
So I decided to try a heritage culture so as to keep on going forever.
I made a starter batch, using 0.5 litres of semi skimmed (2%) milk I had heated to 42° C, sprinkled culture and whisked, left to culture until it looked right, which was about 12-18 hours (I have done this a few times).
Then using culture I followed my usual cold start recipe.
I strain it and it’s a good consistency.

Problem? It’s so acidic, it’s gone beyond tangy, even I find it too much and I like tangy yoghurt.

I am using the NPS cultures, first the Balkan mix and then theBifido mix.
I think it’s something to do with the new cultures and my process, any ideas?

Use full-fat milk and reduce the incubation time for the best results. Any changes to the milk will affect the texture or taste.
It is recommended to boil any milk before use.
Skimmed milk video: https://youtu.be/F2IDEEQIKjM?si=kXJDrBz_vSn5Umto
Boiling of the milk: https://youtu.be/F2IDEEQIKjM?si=ARlmFxxEk1MIudLy
Heirloom culture: https://youtu.be/mk5IdN35Qs0?si=WxSh71V9MRJoqKd_