What do people mean when they say to hold on to bitcoin? If someone wanted to spend 300k usd of bitcoin should they spend this amount as soon as they have it or wait longer until its worth a lot more than just 300k usd? How can this be determined?
You are talking about two different things entirely.
1. Holding Bitcoin is keeping it safe for future sake and future value, for some people it's a good store of value while others choose to make money out of the asset.
2. If someone have 300k its better they buy slowly, maybe start with $1000 BTC every month, this is accumulation and its different from holding, don't get it confused, buying and keeping is where the word "Hold or Hodl" comes from.
If you are accumulating, your plan must be to hold for long term until value appreciate and you can sell or decide to keep holding.