Vocabulary lists sold for IELTS are usually long, rare and useless — a word you have never used in conversation will be used wrongly under pressure, and a wrong rare word costs more than the common word it replaced. What an argument actually needs is a small set of phrases that do a job, used accurately.
Learn these by job, not by meaning, and learn what sits beside each one.
Stating a position. In my view · I would argue that · The stronger case is that · On balance. Avoid "I am totally agree" and "According to me", which are the two most common non-native openings in the test.
Conceding before you disagree. This is the move that separates a discussion from a list. Admittedly · There is some force in the argument that · While it is true that X, Y. A paragraph that concedes and then answers reads as thinking; a paragraph that only asserts reads as memorised.
Cause. leads to · results in · is driven largely by · stems from · accounts for. Note the prepositions: results IN, stems FROM, accounts FOR. These are where marks leak.
Degree. Almost none of your sentences are absolute, so stop writing them as if they are. tends to · is likely to · in most cases · particularly in. "Cities cause pollution" is easy to attack; "heavy traffic accounts for most urban air pollution, particularly in older cities" is not.
Scale and comparison. substantially · marginally · roughly double · a fraction of · far outweighs. Precision in comparison is worth more than any adjective.
Naming the actors. policymakers · employers · households · regulators · local authorities. Students write "the government should" in every essay; naming who actually acts makes an answer specific immediately.
A rule to keep: if you cannot use a phrase correctly in speech today, it is not ready for your writing tomorrow. Take three of the above per week, write four sentences with each, and let the rest wait.