For all your detailed questions, looks like you've got plenty of good, informative responses.
Re:
"IRA contributions: 6000/year which I haven't made this year. Can I contribute this $6000 over the last 4 months of the year and reduce my MAGI by $1500/month?"
Don't forget, not unlike other tax shelters (eg HSA), you can contribute into next year for this. For example, for 2019 IRAs (because of the tax date pushback due to COIVD), you could contribute funds up until July 15, 2020 towards your 2019 IRA deduction.
So far as your bigger question:
"Would you spend more if highly paid?"
Not really. Jenni and I have spent about $40-45K US (as a pair) per year for several years. Our income has varied widely in that period, enough that our savings rate has cracked 75% for years at a time. We built a life we wanted, then started saving for it. We've been fortunate to be happy with that, and fortunate to have increased incomes along with that -- and never feeling like we needed more.
In fact, at one point, I
turned down a $175K salary at just 28 years old.
The thing I value is time.These days, we're long past FI and transitioning into RE with both of us switching to part-time work.
Our spending really hasn't changed through any of this.