Going too fast with a BCS/Grillo is not a factor, the manual shift models have three speed transmissions and the hydrostatic models have infinite speed adjustment.
If I had the Orec I'd get rid of the front wheels and put some skids on it. Front wheels tend to get in the way.
What you really need is a Steiner/Ventrac with a huge monthly pauyment.
I have a 30 year old Steiner that is a true beast.
I have a Grillo 107D with a 21" DelMorino brush mower. I've got hundreds of hours on it. Climbs hills, cuts brush, has steering brakes. I love it. If it can push it over, it can cut it.
One thing it doesn't do is mow in reverse, the pto disengages in reverse.
I've never run a flail mower.
I tried using a Brinley box blade behind my Grillo 107d. It did not work at all on tight gravel. It worked poorly on loose dirt.
Now, the Brinley was designed to be used with a CAT 0 hitch, so I did not expect great results. It does work okay when pulled by my Steiner 420 with CAT 0 hitch.
But...
You could try running the tiller without the tines rotating just to push the seed into the soil. Maybe put a piece of cardboard under the non-rotating tines?