not exactly algorithm, but as you mentioned - curated lists, named, with icons, which beginners can automatically join. But these lists would be instance-local and available only on instances, where moderators choose to offer them (and I hope they would choose wisely)
Very often, the feedback from new users is "but all I can see is empty web page, it is boring". The only place from which they can choose is local timeline (boring) federated "other servers" timeline (very confusing, for newcomers) and then maybe the "Explore" option.
But I think, that having some starter packs, like "Nature photographers", "cycling" or "hiking" would be safe. People with specific interested would be able to relatively easily join some active groups. It could respect the toggle "make this account discoverable for others" - such accounts would be excluded from the "exported" lists.
Besides new users, all instance users would see curated lists besides their own lists (somehow, or maybe in the Explore section) and would be able to mass join curated lists (and perhaps also mass-leave, and also convert to personal list, so they can separated imported follows from their own follows).
Yes, there is risk of creating echo-chambers, but the diversity of such lists among instances would make it safe.