You're almost exactly at the mileage where I've had to replace the starter in both my '95 940s in the last few years. Both cars have a 50/50 highway/city history as far as starter use is concerned. I'm near a wrecking yard and was able to get good used Bosch ones. In all my Volvo experience, you're right, they usually give advance notice for a number of weeks or months. At the beginning, a few more attempts with the key usually urges them into action. As soon as that starts happening, I start packing a length of 2x2, my equivalent of packing a spare without the expense. If it won't start after a couple tries then I use the Fonzi method, open the hood and beat firmly on the starter motor housing a number of times to nudge it off any dead spot. I can usually keep that game up for a number of weeks while I figure replacement. If it was me, I'm not one to replace parts pre-emptively, so I'd probably take my chances on a longer trip, but definitely carry a 2x2 and maybe a hammer. Admittedly, my wife's AAA membership is insurance if a tow was ever needed. Carrying a spare is an option, but replacing that on a trip may not be so simple. Access to the top bolt is difficult at the best of times, including shops inexperienced with Volvos and charging you by the hour.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now