Hello, Norms. Welcome to Just You.
I have been on two river cruises this year and thoroughly enjoyed both of them. They were nothing like what one sees on the advertisements (thank goodness). On the first one (Strasbourg to Budapest) there were 23 of us and on the second (the River Duoro) 17, the rest of the passengers being mainly French. I have also been on a coastal cruise along the Adriatic coast of Croatia (with 19 in the goup) Some of us were single like me, others widowed or divorced and one or two others, like you, who had a partner who wouldn't or couldn't come with them.
To go through your individual points: You don't to have to dress up if you don't want to; most of us wore smart casual to dinner and glammed up a bit more for the Gala Dinner but not everyone did. You don't have to go on all the excursions - if you don't go for the whole package beforehand you can buy them on board as and when you fancy one - and can sit on deck or in the lounge and watch the world go by. On the Rhine/Danube cruise we did sail at night or very early in the morning once or twice but didn't miss much of the passing countryside. I too like my own space and, although I got friendly with others in the group, I needed to withdraw from time to time but as far as I am aware no-one thought me odd. We all ate together at allocated tables but everyone moved round from meal to meal and if you didn't feel up to being the life and soul of the party, no worries, someone else would certainly step in and take over!
I too have looked at the Holland & Classic Rhine trip (with the same river cruise line as the other two - highly recommended) but have already booked my next holiday and have another one in mind for later in the year so another will have to wait. As to keeping the cost down, the cabins on the lower deck are the less expensive and are also the ones included in the JY package unless you upgrade. As far as I am aware they are fine (none of them are huge), the only difference being that you just have a porthole rather than a larger window to look out of. All your meals and drinks (unless you like to drink champagne or cognac) are included in the price so the only additional expenditure would be for tips to the crew and your tour manager and spending money ashore.
To sum up, I'd go for it if you can afford it. You'll have a good time, meet and talk to different people and come home refreshed and perhaps with new topics of conversation to share with your husband.
Good luck.
Ann