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I've picked up an 11-34, so a 14- and a 16- to replace the 11- and 13-
will be a bit cheaper than a whole new casette.I think you’ve got to be careful mixing and matching cogs these days. The tooth profile is designed specifically for the chain to catch the gear either side of it, so for example the 16T in a 12-25 is different to the one in the 11-32 because it has different gears either side. They only work as a set.
You can buy ultegra 14-28 cassettes off the shelf - they are designed for Junior restricted gearing.
https://www.pedalheaven.co.uk/29779/products/shimano-cs-r8000-ultegra-11-speed-cassette-14-28-teeth.aspx