I have an FT-8800, which is the same radio without 6m and 10m. It's a great radio. Based on that I would recommend it.
The FT-8900 cannot do "fancy" APRS stuff. It can be an APRS radio but you'd have to dedicate it to that since it can't do APRS in the background simultaneous to voice on the primary side.
Which brings up a point. If you don't want APRS then the FTM-400 is an awful lot of money for features you'll never use. The FT-8900 does all the regular analog, dual band, dual VFO stuff the FTM-400 does but does not have digital modes nor APRS.
The FT-8900 does add 6m and 10m but those aren't really a substitute for your FT-857 as they are FM only. Very little 6m and 10m traffic is FM, mostly it's SSB or RTTY. IMHO those features look nice on a brochure but aren't really practical. To use 6m and 10m you need a quad band antenna or run a duplexer with multiple antennas. It's frankly a PITA.
I don't do mobile HF. Had it for a while but it's not something I did much (mobile) and when I did get to camp being tied to the cab of the truck wasn't enjoyable. I just take the 857 portable and set up on a table if I want to use it in the field.
All that said, if I was buying an analog radio for the truck right now it would still be the FT-7900. If I had the money (and didn't already have the FT-8800, which is in a box) I'd buy one right now to put on the shelf because it's probably going to be discontinued before long. They already stopped making the FT-8800 a couple of years ago and the 7900 and 8900 came out at the same around the same time, about 15 years ago now.
Alternatively, if I was looking for an APRS radio it would be a TM-D710. The only reason to get the FTM-400 in my view is if you want Yaesu Fusion, their digital mode. I personally use DMR instead, just more actual traffic and users right now. Maybe the Fusion will catch on, I dunno. Alinco seems to be going with DMR, though. Kenwood and of course Icom do D-STAR.