How to state a total for a list of items without any ambiguity?

I did a total of two thousand push-ups and sit-ups last month. At least.

Or

I did loads of push-ups and sit-ups last month: two thousand at least.

I have 6 legs, hands, and eyes.

I have hands, eyes and six legs.

You say, Sometimes, you don't want to throw too many numbers at your reader.

If you really want to tell your reader precisely how long you spent on each task, you may have to! The numbers may look better written as words though.

In context I spent eight hours cleaning, cooking, and gardening would usually make sense: if, for example, you are describing a single day. If you had indeed spent twenty-four hours doing those things then you would surely say, I spent twenty-four hours cleaning....

If the intended meaning is I spent a total of eight hours... then why not use 'total'?

I spent a total of eight hours cleaning, cooking, and gardening

If I graded [approximately] 6,000 assignments, 4,000 midterms, and 2,000 final exams, how do I correctly state the total figure?

I graded [approximately] 12,000 papers: assignments, midterms, and final exams.

or

I graded assignments, midterms and final exams: 12,000 papers in all.

If they aren't papers, you could say pieces of work or submissions or whatever you call them.

How to state a total for a list of items without any ambiguity?

I did a total of two thousand push-ups and sit-ups last month. At least.

Or

I did loads of push-ups and sit-ups last month: two thousand at least.

I have 6 legs, hands, and eyes.

I have hands, eyes and six legs.

You say, Sometimes, you don't want to throw too many numbers at your reader.

If you really want to tell your reader precisely how long you spent on each task, you may have to! The numbers may look better written as words though.

In context I spent eight hours cleaning, cooking, and gardening would usually make sense: if, for example, you are describing a single day. If you had indeed spent twenty-four hours doing those things then you would surely say, I spent twenty-four hours cleaning....

If the intended meaning is I spent a total of eight hours... then why not use 'total'?

I spent a total of eight hours cleaning, cooking, and gardening.

If I graded [approximately] 6,000 assignments, 4,000 midterms, and 2,000 final exams, how do I correctly state the total figure?

I graded [approximately] 12,000 papers: assignments, midterms, and final exams.

or

I graded assignments, midterms and final exams: 12,000 papers in all.

If they aren't papers, you could say pieces of work or submissions or whatever you call them.