Use your bank account for day-to-day needs. Use a brokerage for your investment needs.
Betterment is essentially a brokerage with a user-friendly overlay. Once you transfer the money in, they will automatically invest it based on your goals and risk settings. But underneath, they're buying and selling stocks and bonds. If you use that account for day-to-day expenses, you will make your accounting very complicated. You may be selling stocks at the wrong time, which means your taxes increase or you may be selling at a loss.
Keep your investment transactions to a minimum. On your Betterment account, this includes adjusting your risk allocations. Once you have money invested, changing that slider can increase your taxes and/or cause losses. Don't do it lightly.
Pay yourself first means to automatically transfer your investment allotment out of your day-to-day checking account so you don't see it anymore. When you reach a low balance, you stop spending.
Even if there were no tax and investment impacts, using the brokerage account for regular banking would be a bad idea. You'd see the big investment balance and assume all of it is available to spend. That negates the psychological advantage of pay yourself first.
Therefore, keep two separate accounts. One is for the future, one is for today. Automatically transfer set savings for the future, then conduct daily expenses out of the today money.