I completely agree. PG has so little focus on adding visual parts to this game that I'll take anything at this point, but they could have done so much more - even with brands that they have licensed!
There's plenty of kits missing from brands like KRC Japan, Battle Aero, ADRO, Chargespeed, Varis etc. It's a real shame. We can only hope they continue the trend of adding body kits in updates like in FH5, but do it far far more regularly in FH6
This is the list of 14 Forza Designed kits in FM2023.
2019 Aston Martin Vantage
2016 Audi R8 V10 Plus
2018 BMW M5
2016 Dodge Viper ACR
2019 Ferrari 488 Pista
2016 Ford GT
1993 Ford Mustang Cobra R
2015 Lexus RC-F
2016 Mazda MX-5
2017 Mercedes-AMG GT R
2016 Mercedes-AMG C63 Coupe S
2017 Nissan GT-R
2019 Porsche 911 GT3 RS
2015 Subaru WRX STI
Of those 14, 12 models are in FH6.
Would be great to see all 12 make it - we desperately need more customization in anyway possible.
I actually quite like a few of these kits for their subtlety as you point out, here are some photos of a few;