I have trouble to get contracts as a freelancer. In every case it ends up with the company wanting to hire me and not rent me as a freelancer. The main argument is something like "this is an extreme important role for our company and we want this knowledge to be in house".
What is the best way to make sure they will rent me as a freelancer?
If your services as a freelancer are valuable, it makes sense for the client to prefer to hire you on a permanent basis - so you cannot really blame them for trying.
All you can do is inform them that you are not available for permanent positions. This might get you fired or not hired - and only you can evaluate whether this works out financially for you.
Perhaps you could propose a freelancing role where you mentor an employee to eventually take your place? This seems self-destructive, but if done well - the client might find similar roles for you.
Offer to sign a Non Disclosure Agreement
Maybe the company is reluctant to ask, risking offending you and souring the relationship. There are many sites (1, 2, 3, 4) which offer free templates. This will protect privacy and confidentiality concerns.
Offer training to employees and ongoing support
That gives you more work and a potential source of future revenue.
Explain that as a freelancer you charge X dollars per hour and have the responsibility for a variety of work, along with existing clients who rely on you (demonstrating reliability and loyalty).
That way they know that they'd need to make you a much better offer that you couldn't refuse, offer you a larger role in the company than they had initially planned (both involving an even greater expense), and allow you time to work on external projects; possibly limiting overtime or allowing you an hour per day to take calls.
Many companies would realize that hiring you as a consultant would cost less than thinking that they could hire you full time (for an uncertain duration, as far as you're concerned) for a rate lower than your hourly rate.
If they offer to let you work part time at your full rate and continue your business that might be worth considering, over what seems to be an offer to fold up your business and work at one place; probably for less, and likely not in charge of the division.
Look at the offer to hire as an opportunity to pitch your own idea rather than an awkward situation where you reject them outright. Explain why you feel your current situation is advantageous, and ask what they'd offer to improve it.
As stated on a previous post, propose using you as a 1099 if they are having a difficult time finding a FTE, where you can complete what is needed until they hire a FTE
