I've been using one of these https://smile.amazon.com/gp/product/B08JZ5FVLC for about a month on one RPI 3B+, just added two more to two RPI 4s. They've been working very well. Previously, I have used these https://smile.amazon.com/gp/product/B07NTXSJHB for a little while, but traded up to a MEAN WELL RS-15-5 for 120VAC to 5VDC switching for over a year. No problems.

I usually buy micro USB and USB-C connectors, some flexible silicone insulated wire, and solder everything.

And I always run power to the USB input.

As the other comment mentioned, you'll want a 5V regulator as there as several issues with just using resistors to step down the voltage. You could get a linear regulator which would be a bit simpler but less efficient, or buck/step-down converters are probably a better option. Just looking at the first few search results, there's a pack of 10 on Amazon for $12 that can be configured for a wide range of output voltages: https://www.amazon.com/Weewooday-Regulator-Voltage-Converter-Transformer/dp/B08JZ5FVLC/

Just be aware that LiPos should be balanced/balance-charged so you will need to buy an appropriate charger if you don't already have one. Also, you need to make sure not to discharge LiPos too far; opinions on the exact values vary, but definitely don't let them go below 3V per cell (6V total in your case).