I know a system like zendesk might handle this but it seems a little heavyweight for personal use.
Does anyone have any suggestions for a software solution to this that doesn’t cost an arm and a leg?
The Reply-To header is usually used with one address, but in fact multiple addresses scan be specified.
When sending an e-mail to the "reply-all-averse" individual, put yourself and your wife into the Reply-To header.
When the user tries to reply to the e-mail, their mail client should stuff those addresses into the To: field, so unless they edit that, the reply should go to those addresses.
There is the risk that the user's e-mail client doesn't handle multiple addresses in Reply-To (the developers didn't know about this, or else didn't test it).
If this is just for two people, you could create a mail alias that expands to you and your wife's addresses, and use that as the Reply-To target.
To avoid reply all, use mail merge.