a good idea/option. Given the spare tube is butyl, most regular - not superlight - tubes will stretch plenty enough to take up the small extra space in a 28 or even 32. But the difference in deflated 'size' & weight, compared to the next size up is quite a bit - allows more space for other stuff in small saddlebag.
Been running TPU tubes for about a month on 1 road bike, 23 & 25c. Had my 1st flat yesterday - it's now thorn season out here... TPU definitely needs a higher level of handling, when working with tube or tire swaps and patching... more on this later.
I have tubeless on my gravel and mtb, and do see a nice ride improvement, but not so sure about 23 & 25c for road. At this time the tire selection is limited, compared to tubed tires... I look for good performance tires, but $70+ per is not in my budget...
I have an extra Set of HED wheels lying around... I might make them 'Tubeless' and experiment in the future. 'Tubeless' on a wheelset with narrower than 21 internal, just doesn't make sense to me... A big part of the improved ride and lower psi is due to the advantages of wider internal rim widths (21, 23 and now even 25...).
Ride On
Yuri