How do DeFi projects such as Uniswap or Synthetix make money to cover their development costs? Would you siphon a small amount of ADA each transaction or is it more common to create your own token and follow an ICO-style model?
You are indeed right. I have seen plenty of DeFi projects get their funds by offering a 2017-style ICO model. YaySwap (not an endorsement, just an example) is one of these projects that takes ADA and plans to distribute their YAY token.
Uniswap also takes a fee. Then there's the genesis allocation to team, investors etc.