I have faced the same issue. 5G coverage of Pixels are very bad and if it is for eSim, the problem is too big. Avoid using eSIM for 5G network. Primary / Physical Sim is better for 5G.
Another reason Airtel's 5G is having better coverage is "most of the times Pixels connect to n1 or n3 (depending on circles - whichever spectrum is having min 15MHz of bandwidth) and not n78 band and just shows as 5G.
Best Solution is using 2 separate phones (especially Jio to be used in better phone) like me. If it is not possible, just switch eSims - Airtel to eSim and Jio to Primary / Physical Sim. But still 5G coverage of Pixel 6a will suk for sure. If 5G is NOT a must and you want better coverage and as a result better battery life as well, just select Prefered network to 4G /LTE, instead of 5G.