A contract must be as formal as possible.
You should be ready to sacrifice everything not covered by a contract.
While my plan is to make it informal enough for the client to not need a lawyer to decipher it, I'd still want it to be enforceable.
Sorry to discourage, but I really think you are willing two opposite things.
A contract is a tool used when "everything is bad." It's like an airbag in a car. It can't be both reliable and not hitting on face in case of an accident.
When "everything is good", there is no need to even look into the contract. You only need it when there is an evident conflict of interests.
The major problem in any relations is ambiguity, when two parties assume different things. A contract solves those ambiguities. So, the more things a contract covers, the better it serves its purpose.
In worst case, you are taking the contract to a lawyer. With a badly written contract, even a good lawyer may not help, no matter if you are completely right.
would it be worth it to have two templates depending on the client?
Yes, but it's not about "depending on the client". It's about how much you are ready to lose in worst case.
As per myself, when I'm doing something for my old partner, I don't need a contract at all. Even if they claimed my work is bad, I'm willing to sacrifice my entire effort to save the long-term relations.
OTOH, if I'm doing something for someone I'm not yet familiar with, I would try to make it as much formal as possible.
Is there a particular guideline to how formal/informal a contract should be?
This site has a plenty of real Consulting Agreement contracts. Don't panic, they are not that difficult to read.
In general, you are pretty much close with 7 major sections. @ckpepper02's answer has better names for everything. I would only add:
- Termination - terms on which the contract shall be broken by either party;
- Governing Law - defines specific local laws to stay "over" your contract;
- Intellectual Property and NDA (may be a reference to a separate document);