This is a question about the use of 要 yào as a verb to indicate immediate future ('going to') and its common complementation by 了 le.
要 is a confusing word for learners because of its various meanings. It is usually introduced in beginners' textbooks with the meaning 'to want', which is fine when used as main verb with a noun phrase, as in 我要一杯茶, 'I want a cup of tea'. In this case, I think it is synonymous with the two-character version 想要 xiǎngyào, so we can equally say 我想要一杯茶, with no major change of meaning (maybe just a softer tone, 'I would like' as opposed to a blunt 'I want', in the longer form).
But when used as an auxiliary verb, followed by another verb, 要 can also indicate a future action, a bit like 會 huì or the formal 將 jiāng, but more immediate (I think the subtle difference is similar to 'be going to' v. 'will' in English). This is clear in sentences where there is no possible interpretation in terms of wishing to do something, as in 要下雨了, 'it's going to rain'. In this use, it is often complemented by a final 了.
But where do we draw the line between 'wanting to do something' and 'being going to do something'? I'm aware that these meanings can overlap to a certain extent. If I say 'I'm gonna play football with my friends this evening', I'm both stating an intention and something that I want to do. But it's also possible to be about to do something reluctantly or, conversely, to want to do something that is, however, not possible.
Following on the 'I want a cup of tea' example, consider the following two sentences:
Should we understand these sentences as 'I want to drink a cup of tea' and 'I now want to drink a cup of tea' (assuming a change-of-state 了 in 1b)? Or simply as 'I'm gonna drink a cup of tea'? Does the use of a final 了affect whether 'wanting' or 'going to' is implied?
Now let's suppose I want to say 'I'm gonna pay a lot of taxes this year'. This is clearly not something I want to do. Are the following translations fine?
Does the final 了 in 2b change the meaning? A final question: would the two-character version 將要 jiāngyào also work here?