I'm not advocating it is a literal thousand or an undetermined thousand. I'm arguing the grammar doesn't demand it is an undetermined thousand years.
Saying "thousand years" (without an article) in English is awkward and you might get someone asking you what you mean or correcting you. I get your logic I however don't think it translates well to English and you would have to make a more compelling case with biblical examples.
For example are there any unambiguous references where "thousand" absolutely means a literal one thousand in the bible? Can other numbers be referenced the same way like "hundred" over "one hundred". What do translations favour? and are their any cases where translations are more explicit? Translations are translated by Greek scholars and if the overwhelming amount of translations show a specific reading then an argument can be made to support that reading.
I don't know, I haven't checked those things out, but I think it would make a more compelling argument then what you're making, if nothing else it helps to build your case. To me the context is a greater factor into if it's a literal thousand or undetermined so even if translated correctly as "a thousand years" it still might not be literal and I'm not sure we really can answer this with certainty.
1 Samuel 13:2 Saul chose him three thousand men of Israel; whereof two thousand were with Saul in Michmash and in mount Bethel, and a thousand were with Jonathan in Gibeah of Benjamin: and the rest of the people he sent every man to his tent.
What about an example like this?
Saul chose him three thousand men of Israel
whereof two thousand were with Saul in Michmash and in mount Bethel
and a thousand were with Jonathan in Gibeah of Benjamin
How could a thousand be meaning a plural of thousands or even an indeterminable amount in this example?
1 Samuel 25:2 And there was a man in Maon, whose possessions were in Carmel; and the man was very great, and he had three thousand sheep, and a thousand goats: and he was shearing his sheep in Carmel.
and he had three thousand sheep
and a thousand goats
It seems to me that a plural of thousands should mean 2000, 3000, so on and so on. If a plural of thousands were meant, assuming there is such a thing, the passage should have said something like this instead---and the man was very great, and he had a thousand sheep, and a thousand goats
Ezra 1:9 And this is the number of them: thirty chargers of gold, a thousand chargers of silver, nine and twenty knives,
thirty chargers of gold
a thousand chargers of silver
nine and twenty knives
Is this literally meaning thirty chargers of gold? Is this literally meaning nine and twenty knives? Whatever the answers to those are has to apply in the same manner to a thousand chargers of silver.
Isaiah 30:17 One thousand shall flee at the rebuke of one; at the rebuke of five shall ye flee: till ye be left as a beacon upon the top of a mountain, and as an ensign on an hill.
Here's an interesting passage I have brought up before. Some argue that if it had said one thousand rather than a thousand in Revelation 20, this would indicate a literal thousand is meant. Here's a verse that does have a one in front of thousand. Does that make this thousand a literal thousand, then?
Ecclesiastes 6:6 Yea, though he live a thousand years twice told, yet hath he seen no good: do not all go to one place?
To this day not one single person has lived an entire thousand years. How can this not be meaning a literal thousand years where twice told apparently makes it 2000 years? But if we don't even have a clue as to how much a thousand is, what exactly would it mean to live an indeterminable amount of years twice told? What point would that be making? If a thousand by itself can mean a plural of thousands or whatever, why not simply say---though he live a thousand years, yet hath he seen no good. If that can mean 2000 years, 3000 years, etc, it would cut out the need for adding the part about twice told.
Numbers 35:4 And the suburbs of the cities, which ye shall give unto the Levites, shall reach from the wall of the city and outward a thousand cubits round about.
Numbers 35:5 And ye shall measure from without the city on the east side two thousand cubits, and on the south side two thousand cubits, and on the west side two thousand cubits, and on the north side two thousand cubits; and the city shall be in the midst: this shall be to them the suburbs of the cities.
Since these things involve measuring, how is it possible to accurately measure something if it can't even be determined exactly how much a thousand might mean?