28’s on Propel 2018 Advanced Pro?

Different tyres sit at different heights, you might want to have a check on https://www.bicyclerollingresistance.com/ and check to see which tyres come up with a lower profile. Additionally some 25s on a wider rim are coming up closer to a true 28mm, the conti 4seasons and GP4000SII were measured by the guy at rolling resitance to be 27mm for both for the 25mm tyre at 100psi. The height was 24mm for the 4S and 25 for the GP, this increased to 29mm on the 28mm GP so would unlikely to fit your propel.

Also you could take a look at maybe just changing the rear tyre out and possibly fitting a deeper reach brake, this is basically what they do in the cobbled classics though some teams are going the disc route (though finding that actually causes them other problems).

I run a 27mm Veloflex tub on my KTM carbon and a 25mm conti comp on the front, the 27mm is dead on 27mm and just fits under the arch of my DA9000s, apparently the 9100 variant has a little more space though no-one seems to have said how much extra that is.

Clearly the higher the pressure the higher the tyre will be so trial and error is the way to go, maybe ask your LBS for some knackered/well worn 28s that you can fit to see what if it might work.

GL

It depends on your rims too, whats the internal width of your rim (if you dont know do a search for your wheel / rim online)

For example one wheel Ive got has a 21mm internal width and a 25mm tyre measures 29mm on it, whilst my other wheel has a 17mm internal and a 28mm tyre measures 26mm on it!