Business emails going to spam: where I would start
I'd start with a message that failed and a list of the systems sending for your domain. Your mailbox and website form may be using quite different routes.
Get one useful example
Find out whether the message bounced, reached spam or never appeared at all. Save the delivery report if there is one. Ask which receiving provider was involved and whether the problem affects everyone or only some recipients.
A message from your mailbox and a notification from your website may have different senders behind them. I would test those separately before deciding the whole domain has the same fault.
List who sends using the domain
Include the everyday mailbox, website forms, shop receipts, newsletters and any accounts or booking software. An old service may still send messages even if somebody has forgotten it exists.
The authentication records need to fit that list. Copying another business's DNS settings is a poor starting point because their sending systems may be different. Keep a copy of the current records before agreeing a change.
Understand what the authentication checks do
SPF identifies permitted sending servers. DKIM adds a domain signature that a recipient can check. DMARC adds policy and reporting around authentication and the domain shown to the reader.
These checks help receiving services judge whether a message is authorised. They do not guarantee that it reaches the inbox. The receiving provider also makes its own decisions about the sender and message.
Be careful when tightening the policy
A strict DMARC policy can cause legitimate mail to be rejected when a sending service has not been set up correctly. I would establish the sender list and check reports before making that policy stricter.
The aim is to protect the domain while keeping authorised business messages working. After a change, test the routes that matter, including form notifications and automated receipts. A test from your normal mailbox alone does not cover them.
Know what a repair can promise
A useful diagnosis explains the failing route, the change made and the checks afterwards. It should also say whether further monitoring or work with a mail provider is needed.
I cannot promise that every recipient will put your mail in their inbox. I can look at the evidence, check the configuration and explain what we can change. Send the domain and a failing example to start that conversation.