@Ronnie ******
Once on the “agent train” it will be difficult for you to get off it as immigration will know you used an agent and will probably require you to do so again.
You can jump off that train by leaving and entering on a “clean stamp” or new visa and DIY your extensions.
Depends on what the agent is charging you as to whether you’d want to keep using them. 14k per year (for some agents) isn’t an excessive amount to charge based on they do everything for you and save X number of visits to immigration and the time taken for those visits.
Many here will advocate DIY because it’s only 1,900 to do so. Your call really.