Small businesses should take a leaf out of big business book for this sort of thing.
Handbooks explaining how to cover a person's role.
Defined and structured job roles - not simply oh that's one of 'Anna's areas'. Don't have 'Anna' and 'Sue' have 'an administrative manager' and a 'HR assistant' etc. Make the roles clear, and don't let a person's work ever only their business.
Hire temps for a full year, utilise job sharing, hybrid working, flexi-time, TOIL etc etc.
Yes, maternity leave can be difficult for companies, but larger companies don't let it destroy them. It's not about the size of the business the woman works in either. Think of, for example, a small roadside Greggs. Unlikely to employ more than 10 people. If one full time worker went on maternity 10% (likely more given some will be part time) of their staff has gone! And yet I guarantee it doesn't throw them into hell.
Have a succession plan with all your employees - identity the best ones to be able to step up - even if only for a year. If a bottom rung employee leaves for maternity, hire another, temp or permanent, and look to grow.
If an employee having maternity leave causes a problem, that problem lies with the business, and their solutions should be in place before anyone so much as announces a pregnancy.