Thanks again. To close the loop on this, we ended up flying via DEL. Our Indigo flight arrived at T2, which is walking distance to T3.

We had booked the Atithya service mentioned by Oliver2002 via our tour operator Peirce & Leslie, who were fantastic incidentally.

The Atithya departure service only kicked in at the entrance to T3; they help you with check-in and emigration, then escort you to the lounge (we had access to the Encalm Prive lounge via Lufthansa, don't know which lounge they use otherwise). Unfortunately they initially dropped the ball and forgot to come get us in the lounge to assist with boarding for some reason; I went to the lounge reception area (the lounge is run by the same company as the one that operates the Atithya service) to complain and they found someone to escort us to the gate via buggy within 10 minutes.

We also used the Atithya service on arrival in Delhi T3 at the start of our trip. That was really helpful for my travel companion as well, since they pick you up with a buggy right at the end of the jetway and accompany you all the way whomever you're meeting at the terminal exit.

The buggy rides were a real benefit, as T3 is massive and both our departure and arrival gates were at the far reaches of the terminal. So all in all, would definitely recommend the Atithya service if traveling with someone who has difficulty walking long distances and/or you have lots of hand luggage.

Thanks again to everyone who provided feedback and especially oliver2002!