Getting the basics right is the first step - information provision.
I cannot be the only one who finds that missing timetable information at bus stops is becoming more frequent. Out-of-date and a diminishing number of spider maps online, and a lack of a network map(s) utterly irritating. I previously posted about occurrences of bus stop route flags not being updated a week after route changes, which I found another couple of instances of just pre-pandemic. If you want to promote bus travel, missing maps at bus shelters do not help.
As
@johncrossley says, London buses do not need extensive promotion, as the brand is the network, connectivity, and convenience. If you make it easy for people to find out what bus they need, people will use them. Sort out issues with journey speed, as no amount of marketing will help if people have a bad experience with a slow journey.
While it is a contracting network, frequency on most routes is still better than other cities and fares more than reasonable. Get the basics right and it is still a very good operation.