it is possible to have a large population, high morale and high tax rates.
Please define first what you consider those to be.
If you consider a population of 20b to be high, then that is perfectly achievable. WIllythemailboy, however, was talking about a population of 32.2b in his post.
I consider an approval value of 75%+, and a tax rate of 69%+ to be high.
A tax rate of 69% reduces morale by 96%. In order to keep an approval rating of 75%+, you need to have a rating of at least 171% before the tax penalty gets applied. On a high-PQ world with a population of 32b+, your morale bonus therefore needs to be at least 1510%. If it is a low- or mid-PQ planet, your morale bonus needs to be at least 1610%.
You can get your Morale ability to about 147% via racial techs and Trade Goods, if I'm not mistaken. Add in another 20% via race customisation. With six fully decked out Mining starbases, you can get another 234% (360%, if your playing the Thalan and have all factory techs). In total, that's 401% (527% for the Thalan). You still need another 1109% (983% for the Thalan). That's about 23 Torian Hot Springs (20 for the Thalan).
Considering that one tiles gets used up by the Initial Colony, and you need at least four Intensive Farming improvements to get a population of 32b, the min-PQ required for those planets is 28. Good luck finding many of those. However, this is also a big waste of money, because, if you're able to keep 32b happy, you could go all the way to 100b. The base approval doesn't change once you've reached 25b. However, finding a planet with a high enough PQ to do this is pretty unrealistic.
And playing at cakewalk level makes that all easy to achieve
I thought, we are talking about actually playing the game, not just fooling around. 
