You're free to offer any terms you desire and if people accept them than may you find mutual success, but I've got two comments:

1) People who know what "the going rate" is will not be overwhelmingly enthusiastic about you asking for a board seat given the package deal here. An option on a board seat is, approximately, as expensive or more expensive than a board seat. For example, it's going to cause auto-failures of negotiations with later stage firms who would otherwise be prepared to pay market price for board seats (my SWAG from outside the Valley is "in the neighborhood of multiple millions currently"), because board seats have to be static and scarce to retain value.

You also probably uniquely cause signaling risk because at least some actors are going to model your decision to take or not take board seats like they would themselves choose to take or not take board seats, and come to the conclusion "A prior investor has a free option on a board seat and has declined to exercise it, despite having had full knowledge of the business' deepest secrets for the last year? Wow, that makes my investing decision a lot easier: PASS!"

2) It seems to me that your strategic reason for asking for the option to a board seat is that you desire to take a personal hand in managing downside risk when some startups you fund implode. This implies that you both believe your contribution will help to manage downside risk when startups implode, and that rescuing imploding startups is a great use of your time. Many people in the community would advise that a startup which is imploding is almost immune to correctional action and accordingly valued at approximately zero, and that startups imploding is sort of the model and that your main source of risk reduction is having 7.5% invested in Google 2020 rather than tweaking twenty imploded companies to slightly-north-of-imploded.

Or, to rephrase, if the successful outcome is "We do not get a board seat" then do not ask for a board seat.

1/ For next rounds, we are almost all the time leaving our seat to the new investor (if it's a good/great one. Not if it's a shark VC) More than 50% of our 2010/ 2011/2012 investments raised a Series A, Series B and more. So nope, you're not right. We have excellent reputation in the ecosystem.

We invested in 220 startups. Had this right to join board everywhere and never used it until now but I still prefer to keep this right.

2/ Discordance between cofounders (in case they are not sharing the same strategy) is not always a startup implosion. Sometimes we just need to come and become the 3rd vote who can decide which founder strategy we will choose.

Have 50% of YC investments raised a series A?

The term sheet gives you the unconstrained option to appoint a director. Again: $150k doesn't sound like "appoint a director" money.