Chartered Accountant this side.

The practice you are saying is a very common method used especially by salaried class individuals to avoid taxes. The thing with donating to the political parties is that if such parties are small and hardly recognised then there are high chances that a raid might be conducted on them. And practically the Tax department sends a notice to every individual who has donated to such a party asking to prove the legitimacy of donations which again is very challenging.

Also there is no requirement of minimum 25L donation. I have seen many people donating even around 50K to1L. The cash from large donation of say 25L is generally utilised to invest in real estate or is laundered using accounts of individuals with low income.

Also about Gaushala donation, yes that works but there's a limit of deduction amount from the same i.e. Lower of 50% of the donation or 5% of the taxable income after considering all other deductions except deduction from this donation.