I just ran into the same problem earlier today, so I'll leave this in case anyone else stumbles upon the same problem.

As of 2025-01-16, the only clean way to do this is to create your own implementation of IHybridCacheSerializerFactory and register it in place of DefaultJsonSerializerFactory.

Register the service and your custom serializer factory:

builder.Services.AddHybridCache().AddSerializerFactory<CustomJsonSerializerFactory>();

Create CustomJsonSerializerFactory.cs:

internal sealed class CustomJsonSerializerFactory : IHybridCacheSerializerFactory
{
    private static readonly JsonSerializerOptions _defaultOptions;

    static CustomJsonSerializerFactory() =>
        _defaultOptions = new JsonSerializerOptions
        {
            DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingNull
        };

    public bool TryCreateSerializer<T>([NotNullWhen(true)] out IHybridCacheSerializer<T>? serializer)
    {
        serializer = new DefaultJsonSerializer<T>();
        return true;
    }

    internal sealed class DefaultJsonSerializer<T> : IHybridCacheSerializer<T>
    {
        T IHybridCacheSerializer<T>.Deserialize(ReadOnlySequence<byte> source)
        {
            var reader = new Utf8JsonReader(source);
            return JsonSerializer.Deserialize<T>(ref reader, _defaultOptions)!;
        }

        void IHybridCacheSerializer<T>.Serialize(T value, IBufferWriter<byte> target)
        {
            using var writer = new Utf8JsonWriter(target);
            JsonSerializer.Serialize(writer, value, _defaultOptions);
        }
    }
}

As needed, you may specify other options you find useful as well.

See this question as well, as the conclusion by the OP was somewhat similar: How can I use NodaTime with redis?