How to squash multiple commits from the beginning into one?

I have a repo which I use for personal notes. I use the same repo across multiple machines and multiple OSes. The sync happens using git. Each machine has a 5-minute cron job that both pushes and pulls from the (same) repo - that keeps it consistent across all the machines.

I have accumulated thousands of commits over time and I don't really need commits from over a month ago (I have more than a year worth of updates). So I need to squash say first 1000 commits into one without having to change my entire setup by recreating the entire repository and setting up everything again!

What can I do to squash the first 1000 commits into one thus making the repo size smaller?

Exmaple a tree history: the order is from oldest to newest commit.

We are only able to squash commits newest into a old commit. it means we can squash 1 into 2, or 1, 2 into 3 or 2, 3 in to 4, so forth.

Of course, after squash, commit hash id was squashed into, corresponding to examples above are 2 or 3 or 4, so forth; also will be changed.

(master): m1<--m2<--m3<--f1<--f2<--f3(HEAD)
		  6    5    4    3     2    1

you want to squash (4 commits latest) - 4 3 2 1, you can do this

first:

git rebase -i HEAD~4 (1, 2, 3 merge into 4)

second:

popup appear (vim editor), you have to replace pick with s or squash in (3, 2, 1)

finally:

wq!

if you want to push those changes to remote, you must specify -f

I have provided a script to do this without much hassle, including merges:

https://github.com/eantoranz/git-replay

So.... Check-out a new branch starting from the commit that has the tree that you want as the first commit. Suppose this commit is X

git checkout --orphan new-branch X
git add .
git commit -m "first commit"

Now, use the script

./git-replay HEAD X original-branch

That Will print, in the end, the id of a commit that has a history just like the one you want. Check it and, if you like it, use that commit to set the branch you want there.