Performance line bearings would appear to be the best place to go with an out of warranty Bosch or for that matter Yamaha crank drive motor, as soundwave was quick to say.
I have both a Yamaha Crank Drive Haibike and two rear hub old Oxygen Emate Cities.
For serious off road riding my crank drive bike is superb. On the road it is fine, but I prefer my old rear hub Oxygen Emate.
I have been contributing to a warts and all review of my Haibike since I bought it in 2015 for £1750. However I bought it to be able to ride to work cross country using some challenging tracks and terrain. This it has done brilliantly.
https://www.pedelecs.co.uk/forum/threads/haibike-sduro-hardseven-sl-2015-yamaha-7-month-1600-miles.22644/
I quickly found that it wore the drive train out an order of magnitude faster than my rear hub bikes. I increased my gearing and adapted my riding style and get around 1500 miles out of a chain and rear cassette and change the chain ring at around 3000 miles. Last rear 9 speed cassette cost £15 from amazon and a kmc chain £10.
I am now in my sixth year of riding it, had a second motor fitted under warranty right on the two year point free of charge, and I am using the original battery and have completed 12,216 miles so far.
I am on my third rear wheel. I carry full ortleib panniers, am heavy 115kg, and ride it as it should be off road which can include coming of sandstone steps at about 20 mph to land on the track below. I love it.
My Oxygen Emate City cost £1400 and was bought in 2011 and is still going. Had a new motor wheel fitted right on the two year warranty period free of charge. Has just had a third motor wheel fitted. The second motor wheel broke spokes which can be a disadvantage of rear hub driven bikes but I kept changing them and generally checking and adjusting the spokes until it stopped but it took a while to get to that point. The Oxygen has mechanical disc brakes that are fine on the road but require a lot of adjustment and The Haibike has hydraulic disc brakes which are much better. The Oxygen is on its third battery. I love it.
The bike dealer I bought the Haibike from has provided very good after sales support. He can change the bearing if it develops play.
However the importer Raleigh offered no help at all to fix the battery/charger connectors when they broke and I had to do it myself. If I had not been able to fix them I would have either had to scrap the bike or buy a new 400Wh battery and charger for £800, no thanks.
https://www.pedelecs.co.uk/forum/threads/yamaha-battery-charger-problem.31092/#post-434202
The bike shop I bought the Oxygen from provided no support at all. Oxygen the company have offered very good support.
The best value electric bike I have bought was a second hand Oxygen Emate similar to the one I own, originally for spars, but I found that it all worked including the battery and I have now covered over a thousand miles on it.
https://www.pedelecs.co.uk/forum/threads/the-tale-of-a-£100-second-hand-electric-bike.33079/#post-467114
Bikes like The Oxygen can usually be fixed yourself quite easily and cheaply as they use generic Chinese parts. Bikes using Bosch and Yamaha motors cannot, though hopefully people like the bearing man will become more prevalent to fix out of warranty motors.
If you want to go off road onto challenging tracks in my opinion it needs to be a crank drive bike. Riding on the road my preference is a simple cadence rear hub drive bike, but I can see how someone else might prefer the natural feel of a crank drive.
A lot of that feel comes from the control method rather than the type of motor, torque sensor or simple cadence sensor and these can be on either a crank or rear hub driven bike or probably any other style of motor for that matter.
The truth is out there and like everything else it is not a case of all crank drive being bad and rear hub driven bikes being good or vise versa for that matter.