@Raven87 not sure how rigid the rest of your layout is, but take a look at the length of the straight section where you plan to add the switch and see if there's any room to adjust. For instance, in the 20" scenario if you could adjust to 19.75", then you could use 3x of the 1 3/4" fillers instead of 4x of the 1 3/8".
I agree it's not fun to use a lot of small fillers to get a certain straight length, and it cal make for a bumpy ride as well. I have 0 issues with the 1 3/4" fillers, but the 1 3/8" ones are pretty hit or miss - the outer rails rarely seem to be level and I've had limited success trying to level them myself. Whenever possible, I try to avoid using too many of them or at least space them out so it doesn't look my trains are on a 5" "gravel road" track section.
However this is pretty much my only complaint about Fastrack, so I'll happily keep using it for the foreseeable future. I love the CC switches, built in roadbed, lift out extended truss bridge, and now that I have built up a pretty large collection, the ability to reconfigure my layout as often as I'd like. The pros definitely outweigh the cons for me.
