It seems that you want a build that can handle both xbowcav as well as playing a shielder on the ground. You must realize that 1h's don't hit very hard and without further dedication into 1h you'll just not be very effective. Shields are nice early on when you're still learning because it allows you to survive a hair longer, but you honestly should get used to manual blocking because of how long you're going to have to be fighting to kill someone with that low str and ps with a 1h.
I would advise either going for a more dedicated xbow-cav build such the Zapper 15/21 build (or even one of the minimaxed Malaclypse builds), or to go for a build with much more 1h dedication that will not have nearly the accuracy on horseback as a dedicated.
Trust me on this, I'm always trying to hybrid my builds to give me as much playstyle as possible, but I really notice when I don't hit for much damage that I'm in fights way longer.
Edit:: If you really want to have shield, throw away much of your xbow ability.
21str, 15 agi
7PS
5Shd, 5Ath, 5Rid, 2HA, 5WM
1001h, 121Xbow wpf
Move around a couple skill points as needed. You would need to have more oomph in your 1h hits.
And another edit: Maybe try a support/hoplite xbowcav hybrid? You wouldn't hit as hard, but you could get a bit more xbow wpf and try to back off to allies when your horse inevitably gets shot down.
18str, 18agi
6PS
6Shd, 6Ath, 5Rid, 6WM
100Pole, 133Xbow
You could lose an Ath point and maybe a shield point to put back into HA if you want to be more into the xbowcav, but you'll have a harder time escaping the angry horde of horsehunters