"This tea tastes bad."
Correct. まずい at its core means tasteless and can be used additionally in situations where what is being said is tasteless or a situation which is grim. As regards taste of food, however, まずい would generally refer to either bland food or food without a flavor that one appreciates favorably.
AはBです is the standard formula for describing something about A. Using が instead of は would show contrast, in this case between different teas.
このお茶はまずいです。This tea tastes bad.
このお茶がまずいです。This is the tea that tastes bad (not the other one).
If there is only one tea within the context of the conversation, が would not make much sense, as there would be nothing to contrast it against.
が vs は
If you say このお茶がまずいです, you're laying emphasis on このお茶. So it'll be like "it is this tea, that I do not like." You can imagine it as a response to someone asking you what tea you like.
If you say このお茶はまずいです, you're laying the emphasis on まずいです。This will be understood as "For tea, I do not like it". The information you're conveying, is that you do not like the tea.
Explained much better by https://youtu.be/FknmUij6ZIk
Just a tip: you might wanna redo the first lessons where duo teaches hiragana, because it shows a lot of romaji when introducing the characters, such as ocha for small ゃ, kyo for small ょ, kyu for small ゅ and natto for small っ, and the same for katakana, so you can get used to what to type when you need them.
The DESU? It is pronounced DES in the Tokyo dialect and most dialects as far as I know. HOWEVER, having heard plenty of Japanese songs, it can sometimes be pronounced as DE-SU. I believe it is a regional thing. In some dialects, the SU is pronounced in this word.
There are words in which it is pronounce so I believe that if the SU comes in the beginning of a word it can be pronounced... You should look it up, but I definitely believe so (for example, 住んでいます, sundeimasu, the su in the first mora is pronounced as SU not S)...
My native language is spanish so I use the romaji to guide my pronunciation and it works well. I will tell you that DAY as in "Today is Sunday" has an extra vowel that will mess up your pronunciation of desu.
It is DE as in DEck. And if you do pronounce the whole SU, it would be as in SUrreal.
Certain words such as お茶, お金, and ご飯 are the default. While they do have the 'honorific' attached, that usage is now so commonplace that to omit them would look unnatural and too casual.
Aside from that, just because 'this tea' tastes bad doesn't mean that the respect for tea itself or for tea as a concept is diminished.