Haven't done this recently but about 6 weeks ago I was in that area and there were plenty of foreigners doing the loop on small "step-through" bikes. The first part from Thakheak to Nakai is all paved and from Nakai the next 35 km is gravel so that is no problem. Even though there might have been some rain I don't think that it will cause any problems at this point in time (at the end of the rainy season it will be muddy). Anyway, they are upgrading the road from Lak Xao to the south so part of it should be already quite good.
From Thakeak to Lak Xao is 150 km so you should be able to do it easily in 5-6 hours and I have met people who did the whole loop (Thakheak to Lak Xao and then on to Thakheak) in one day but most people take 2-3 days including a visit to the Konglor cave.