If the only time you don't use a horse is in siege or siege strat battles that build still seems pretty good. Might do 21/21 and get the extra point for ath if your on the ground a lot though. You wouldn't really need it in strat sieges though, so for your purposes your existing build seems pretty good.
The other thing is that you might not need that much PS from horseback with the increase in agi builds. I don't know how it is in EU though. 18/21 might be a good idea too. Leave you a lot more skill points for IF and WM, or more riding.
Str:21
Agi:21
PS:7
IF:2
Shield:5
WM:4
Riding:5
Ath:7
Also the character builder is only outdated wpf wise, so you could still look at skill point allocation with it for the 18/21 build.