Last time I checked with immigration (about 3 months ago), you have to do the first 90 days on-prem (meaning you have to go there), and the second one (if you extend) you can do online.
Best advice is to send an email to immigration in the nearest town and ask them directly.
to make it short - if you make your income outside of Thailand, and specially if you're making a good income, you better not go to Thailand. Spend your money somewhere else.
yes, you only pay taxes on the money you remit into Thailand, those are the very specific information provided by the revenue department of Thailand itself. Money earned outside of Thailand is not taxable if it is not remitted into Thailand
Why do you need a visa company? The process is very straight forward, it cost 10k THB, plus flights, visa and hotels you won't get to 20k. the price they ask of you is just ridiculous.
The rental agreement in Thailand is mandatory? How does one rent a place for long term in Thailand without having a long term visa or even without living in Thailand?