"I'm buying six altogether."
Although you might be right in some contexts, I think perhaps you’re adding specificity that the original sentence doesn’t require. Chinese prefers simpler to more complex. So perhaps it’s a good exercise in reducing and focusing only on the essence of the sentence. Complete but concise. This is just a thought, not a definitive answer. You’d need a native speaker for that.
As a Chinese student, there is a specific order to which a sentence in Chinese is put. Subject + Time + Verb + Object is one way to put it, I suggest you check out this website. https://www.fluentu.com/blog/chinese/2013/10/30/chinese-sentence-structure/
You use 在 (before the verb) to indicate a continuous action. Buying as opposed to buy or bought or will buy. 在买 = In the process of buying.
Technically it could be in the past or the future too but you are in the middle of doing it. e.g. 我昨天在买东西的时候。。。= when I was buying things yesterday...
So it’s best not to think of it as a tense. Think of 在 as indicating continuous action and 了 as indicating completed action.