Firstly thickener.
There is no evidence of benefit to thickener, and significant evidence of harm from it.
If you challenge its use, you will find that the rationale slips away and changes. It will be said that it prevents aspiration (you can't tell that but it probably won't), and that it is 'safer' (it isn't at all, it can often be less safe, but who cares about 'safety' in this context?)
If you challenge those things, the next will be that it's more comfortable. That is sometimes (not always) true in terms of coughing, but there are often other ways round that, and people with dementia won't remember coughing or dread it or worry about it. It is much less comfortable in terms of how fluid feels in the mouth and how long it sits in the throat.
If you challenge that they will probably say it's the speech therapist's recommendations, which they must follow. True that it's the recommendations (though bear in mind that it's still the patient's choice, or the attorney's choice for them, or a decision in their best interests. One profession's advice is only one piece of advice).
If you go back to the speech therapist, talk about what they've seen and what makes them recommend these things. I hope you will be able to have a sensible discussion and get the recommendations changed. If not, ask for a best interests meeting including the GP.
Examples of possible alternative things a speech therapist might say: normal fluid at risk of aspiration, coughing and death; normal fluid from a teaspoon only at risk; water only at risk.
If anyone mentions 'feeding at risk' which they should, it's a good thing, ask them to write down what risks they are talking about. You cannot agree to at-risk feeding without knowing what risks you are accepting.