IMO, it might pay you to find a pistolsmith who's familiar with the design and have them take a look at it.
If what you've already done hasn't had any effect, and your pistol shows similar results in other hands, then there may be something else at work that needs to be addressed. Hopefully, something less drastic and costly than replacing the barrel.
It could be something which minor refitting or a rework for different sights would resolve. Without examining it personally, its impossible to guess what exactly might be necessary.
Finding a model-specific DIY sight replacement solution to try instead, fixed or adjustable, may be difficult if not impossible. A quick browse through the Midway catalog doesn't list anything for an Astra. Brownell's may have something, but I don't have their book at hand.
At the very least a 'smith could identify the precise source of the problem for you. At that point you'd have the option of deciding whether the cost to correct it is worthwhile to you or not.
Hope this is of some help.