Well, ID4 is already on sale so not sure why you stated 2029
Initially I thought the same way like you, i.e. the natural conversion would be Tiguan -> ID.4 as ID.4 is sort of SUV style.
However there are some 'buts'
1. Tiguan has always been a Golf in a SUV posture,
2. Due to dedicated EV MEB platform ID.3 wheelbase is 2770 mm and surprisingly (or not) Tiguan has only 2681mm which means Tig is smaller inside (proven: there is huge amount of space for your legs at the back of ID.3),
3. I got really good lease deal on ID.3 where there is no decent deals on ID.4 at all,
4. ID.3 is quicker than heavy ID.4

with the same electric motor (204 BHP),
5. ID.4 is only (as of now) available in 1st Edition that is fixed trim with almost no extras to personalise. I don't like grey/brown seats on this version too. However German VW site has all future trims shown and there are some interesting ones.
I may only miss the boot size but I can always buy / lease ID.4 in the future
Anyway below is the real photo of the new car
