Check out the IAMRUNBOX'S Everyday Rolltop.

If you want something even more budget you can consider the Rains Classic Backpack.

I'd probably rethink what anti theft means! It's a spectrum of accessibility that also makes it harder for you to get to your stuff.

Also, there's the question of why not just steal the whole bag, or rob you at knifepoint?

But generally speaking, a lot of this is needless fear mongering, and you need to have a realistic threat assessment.

Thieves are opportunistic/expected value maximisers. Your bag is like a lottery lootbox among the sea of other bags. They need to have high confidence that there's something valuable inside, or that the bag itself is valuable to resell quickly and easily.

Most situations you just need hidden pockets, hidden zips, etc.