Anti-Androgens basically block testosterone, but they might have a minor feminizing effect especially since you're starting at 18 and are still developmental. If you're on an Anti-Androgen, you shouldn't have too much to worry about in terms of testosterone damage. Waiting two years might be slightly frustrating as you may perhaps be stuck looking androgynous for the most part until starting Estrogen. For the most part, I don't think you have much to worry about starting Estrogen late if you have an Anti-Androgen for now.
FFS is really expensive so if they are willing to pay for it for you, you should probably take their offer. However, I'd recommend being on Estrogen for a while before doing FFS. That way you can see what hormones will do to your face and then see what aspects FFS can improve upon afterwards as opposed to doing FFS and Hormones right at the same time and rushing the process.