I think you should just change 100 coins to 99 coins, just like stars, it would make it easier.

I mean you could set 1 coin for item ans still collect 100 coins, but i's harder to count for everyone. I'd suggest in this category to strictly match the "coins for item" with the number of coins you have to collect (10, 25, 50 and 99) so you can see on the screen the progress of the speedrun at each coins collected.

Louis just did this in their 10 coins grasslands speedrun... set 10 coins for item and went for the speedrun

And if you try multiple coins goals at the same time (f.e 10, 25 and 50), you can just set 50 and make the 10 and 25 a split in the Livesplit app like in the picture.