You don't need Workspace to use the Gmail UI with your custom domain. All you need is a regular free @gmail.com account. First switch your domain to an email hosting service that provides SMTP and IMAP, you probably get one free from your domain registrar. In Gmail settings for your @gmail account, add your custom domain as an alternate address for sending mail, using your SMTP server. Also configure Gmail to fetch mail from your IMAP server. Voila! Gmail is now a frontend for your custom domain, absolutely free. You can continue to use your existing custom email addresses, and you can forget about Workspace forever.
If you have purchased content such as Play Store apps that are tied to your Workspace account, you can downgrade it to a "Cloud Identity Free Edition" account, and continue to use it. This is a pain, and I think they are finally working on some kind of migration tool to move that content to another account. Hopefully that happens soon.