There are a number of additional points that apply to non-executive directors and in-house roles generally. There are some material risks you should consider before accepting a NED role (as well as those above regarding additional roles generally).
As a director you owe director's duties to the company as a whole. This is both at the objective level and (where you have a specialism as you do) the subjective level. So, you will be required to become their de facto legal adviser and will be personally responsible to the company for any breach of duty. It's not the same as being their lawyer in the usual private practice sense, but it's a risk. You'd need to think about whether this extra level of responsibility is worth the risk. If you are minded to accept it, then you should require a comprehensive Directors' and Officers' Insurance Policy (D&O cover).
You should consider, instead of being a NED, agreeing to be an observer. If so, you have no duties, but it's always good to record that in writing before you attend, so there is no risk that you are accepting any duties. Observers can still be paid for their work.
If you accept a role as a NED or an observer, then you should note that your position may well prevent the firm (i.e. both you or colleagues) from acting for the company that has engaged you. This is on account of your fiduciary duties to the company that has appointed you and the information to which you become privy.
If you are in-house, as a NED or otherwise, the client will not be able to recover from the other side the cost of the work you do.
Also, you should note that there is no privilege in any correspondence between directors. So, where you are a NED and involved in the giving of legal advice, you'll need to be very careful to ensure that solicitor/client communications remain as such and avoid any grey area where the solicitor becomes part of the client.
A similar logic applies to other roles you might accept, such as going on committees or chairing associations and entering into (directly and indirectly) any transactions outside the firm with a client. These create a personal interest for you and you need to exercise great care to ensure that it cannot be, and cannot be seen to be, in conflict with your duties as a solicitor, both inside the firm and your duties that apply to you outside of the firm. Entering into any contract with any client personally is not permitted.
Please see the guidance on your duties to register your own interests.