Would like to make a standalone board; advises needed.

I've been using Arduino since 2012 for various projects, starting from blinking LEDs to remote controls.

Recently I got into designing model rockets, and decided to make my very own flight logger to track altitude, speed, position, etc. I could have use Arduino Micro/Nano/Mini for this purpose, but since I would like to have my sensors to be in single PCB as well, I decided to start looking into custom PCB making as well.

I have searched around to learn how to make standalone arduinos, but still, few questions remain unanswered from my research.

First question is; What tools do I need to properly upload a code to a standalone board? Will it be enough just to have ICSP tool to upload my code? Will it be possible to burn bootloaders to my standalone design with ICSP tools? Here is an ISP programmer I found as an example.

Second question is; if I wanted to change pinouts for standalone board, what are the things do I need to configure? I'd like to use ATTiny 84, and although I've seen a tutorial about how to upload a code to ATTiny, I haven't found any tutorial/instruction on how to change pinouts, as I would like to use I2C for my sensors.

Last question is; How small can you actually hot air solder by hand(That is, pick and place by hands/tweezer)? I have a hot air soldering station and would like to use SMD parts for my standalone board.