Can I set a decimal tax rate like 2.5%?

Yes. The Rate box on Settings → Store Settings → Manage Tax accepts decimals up to three places, such as 2.5 or 12.375. Add each tax as its own rate, for example CGST 2.5 and SGST 2.5, and click Add rate.

The tax amounts are worked out exactly, and checkout and the order page show each line with its rate.

One thing to know: the tax percentage printed by invoice templates is stored as a whole number, so 2.5% can print as 3%. The tax amount itself is correct. If that matters, edit your invoice template under Invoice Templates so it shows the amount but not the percentage.