Might be wrong, but it’s likely that only real solution is for the organization to introduce controls that attempt to put distance between the donors and operations of organization. Issue is donors, especially large ones, are very aware that organizations crave additional funds and often willing to bend the operations and positioning of an organization to gain them.
Beyond that, free speech is complex topic, so one for which I am sure I lack an even partial understanding of. I can easily see how hate speech is outsides of free speech in part because it feels without fail it’s only matter of time before someone takes the words to heart and lashes out in violence.
Given it appears only four lawyers at ACLU actually engage in free speech litigation, if true, beyond ACLU brand recognition and legacy funding, likely be worth funding and alternative organization instead of waiting for ACLU to default on its founding principals.
Lastly, ACLU might easily address issue by making their potential case review assessments public and requiring those assessments include disclosure of any conflicting values and beliefs related to the case.
I don't think there is a solution to this, though. If ACLU doesn't get a steady supply of new members, it will wither and die. If it constrains itself to be small enough that the available supply of new members who fully support the existing platform is sufficient to sustain it (even as that platform becomes further away from the mainstream), it will be too small and lacking power for many important fights.
But I think it's best to go directly to the source, which would be the 2018 ACLU memo that triggered the attention and judge for yourself: https://online.wsj.com/public/resources/documents/20180621AC...
To be fair to ACLU, they did backtrack somewhat on all this. Speaking as a member who joined years before this became somehow contentious for the organization, I hope that it'll get back on track, but I'm pessimistic about it - I think the zeitgeist is too contrary, and historically, ACLU did waver at such times (e.g. on defending rights of communists in 1930-50s).