I would advice you to use an agent for your first time. The reason being is the visa application is kind of set up for you to fail, a good visa agent knows the right answers to all of the questions and what to do and not to do and will avoid you a lot of the stress that comes with it
If you searching online don’t go to the car company direct use a car rental search engine - they compete on the prices on those so it’ll be a lot cheaper especially for the drop off in another city
It’s possible to get 30 days on arrival with the option to get an extra 30 days after visiting an immigration office. It’ll depend which passport you have 👍
I’m not a massive fan of big cities so I would rather go to George Town in Malaysia 👍 I’m not sure how long the tourist visa takes from your home country i think all embassies differ. Another fun option is to go to Angkor wat in siem reap
Did you want to go to KL or are you going specifically for a visa? If you didn’t want to leave Thailand you could apply for a visa to cover that amount of time with one extension before you leave 👍