With the help of welcome offers (free bets) on different betting websites, combined with the 'lay' option (shorting) on the Betfair exchange, some users can make a lot of money. There are quite a few services you can use that find the best odds to make the greatest returns.
Yes, that's actually a form of Matched Betting or Bonus Arbitrage that you describe, That was my strategy #1 (see OP). Betfair's 'Lay' betting can definitely help with that strategy cos it helps you take the other side of the bonus bet (with a separate sportsbook) much more easily. Do you use Betfair at all cryptossi, or another betting exchange? I have used Betfair a lot in the past, mostly for soccer, but it's tough.... lots of smart money in those big markets...