I'm the same like you, started late season 16. Now that I got a team that's stable in the division I'm in, I'm expanding my stadium in the following way:
1. I expand all seats with 10%, so lets say you have 5000 seats, expand with 500, with all at a minimum of 1 (VIP boxes).
2. Look how the turn out is, if I sell out 2 times in a row, I expand with another 10%. I also set goals for myself at how big I want my stadium to be at the end of seasons 19 and 20. Don't think too much in short term.
3. If I don't sell out, I drop the ticket price a little bit. At the beginning of the new season increase the price back (new season new chances)