T itself suppresses estrogen production. So no, not needed. Estrogen blockers are only used in trans pre-teens and teens who want to offset puberty but not start hormones right away. In FTMs they are rarely used simultaneously, and the overlap usually only occurs in situations where someone was a pre-teen who started on blockers and will continue with the blocker for a short period of time while they start T. Many MTFs do require an anti-androgen pre-SRS because of the way hormones work in the body.
Testosterone converts to estrogen when your levels are too high. What # that is, is different for everyone. I've been as high as 900 with low E and had no issues, while I know some guys who can't exceed 500 before their E starts to jump.
T converts to estrogen in fat cells. So the higher body fat you have, the more conversion is taking place. It is why you see larger cis men with "boobs" and why larger FTMs may not masculinize at the same rate as someone with half the amount of body fat who started T at the same time.
It took me less than a month to get into the male range, but I had a pre-existing condition that caused high T.
It got rid of my anxiety, made me calmer.
Injections and gel are exactly the same in terms of results and masculinization over time, it just depends which form is more convenient for your lifestyle. I find gel to be inconvenient because I'm very active and thus sweat a lot.