Fix: Outbound business emails sent via Gmail always go to spam

We use gmail as a convenient "front end" for all our business emails - we are a small team of 3 individuals, between us sharing about 10 separate email addresses through which we interact with our customers and B2B queries.

The issue is, when sending from my business email address(es) - using gmail as the client/interface - the email appears to have been sent but the recipient will ALWAYS have the email go directly to junk - no matter who it is.

Eg If I send a single email (using gmail), to a known or trusted colleague; even if replying to an email which they sent to me first
a) if I use my "from" address as info@trailaddiction.com, I know that the recipient won't receive it since it will go to their spam.
b) if I use my gmail address which is associated with the above address (through which we can receive and read all emails sent to info@trailaddiction.com via POP) the email will invariably get delivered OK (but not always).

We have the same problem across all our businesses domains and associated emails (@enduro2.fr, @nzmtbrally.com).

The interim solution for the past 12 months or so has been to only send outbound emails using our gmail address in the "from" field but this gives the impression of an amateur setup so we really would like to fix it.

FYI we do use Brevo to send out occasional marketing emails (perhaps 8 to 10 per year) but we are not spamming and our marketing lists are genuine and of high quality (former customers or people actively signing up for our newsletter via our website).

I have a feeling that part of the issue is an attempt by google to force users to migrate to the paid-for "workspace" service. I have a rudimentary understanding of authentication (it is set up correctly on our Brevo account) but perhaps an expert would be able to improve / fix the problem once and for all. (This was never an issue in the past, for years we have used gmail in this manner without the above issues so I feel sure the root cause must be a policy-change at gmail).