I believe you can do what you want. In VFR map mode you'll find an icon at the lower left that looks like 3 sheets of paper and a curved arrow. Press/click on that. Choose TAC. iFly will try to stay on the TAC no matter your zoom level -- until you get out so far (quite a ways) that it will change over to Sectional or WAC, but if you zoom in again it will resume using the TAC.
Note that on Windows there is a bug, which I reported awhile back. In Android, once you select TAC it will tell you if Sectional or WAC is the "natural" chart for this zoom level and also tell you that TAC is the "forced" chart. This does not occur in Windows, and furthermore, it always says Sectional (or WAC) is the selected chart no matter that you chose TAC and TAC is really the chart being displayed.