Chainring ratios - advice

I have got the Sora triple (FC-3503) not Tiagra, but they both have an inner BCD of 74.  Technically you can go as low as 24 for your inner.  On my Sora i have replaced the stock 30 with a 26 and without having to make any adjustment to the position of the braze on front derailleur (Sora triple) it still shifts perfectly.  Theoretically this makes the tooth difference larger than the stated capacity of the mechs but in reality it is fine.  This means you can keep your 50-39 but improve the your range at the bottom end.  Spa have a range of 74 BCD inners from TA, Stronglight and Spa.  The Stronglight Zicral ones look the best value.

Good to know that 28T will work. I also have FC-4503, the inner ring BCD is 74mm.

Whether it shifts accurately probably depends on the front mech, including whether it can be positioned low enough on the seat tube to shift to/from a chainring significantly smaller than 30T.

You may like to investigate a super-compact or touring chainset like those available from Spa Cycles (e.g. this one & this one).

On 105, a 28 granny definitely works. A 24 granny works if you make a whole shift to the middle ring upper trim position. Suspect 26 works ok.