You can sort this all out yourself as long as you are organized. With 8 people traveling, you should go ahead and make arrangements for things like lodging as soon as you have firmed up itinerary. For sleeper train tickets, you must use an agent to get tickets in advance, and pay the commission. Put order in for those about 1 month before leaving. For other day train tickets, purchase when you get to China (maybe at Guilin station). With 8 people together, there are some situations where it will make more sense from a cost- and time-efficiency standpoint to use a private vehicle (very large van or small bus) and driver. This should be arranged in advance either through the origin point guesthouse or hotel, or through an independent local travel agent.

Itinerary: STRONGLY suggest that you dump Shanghai/Suzhou off of this plan. Geographically they are outliers and will add another layer of cost, logistics hassle, and frazzledness to this plan....and IMO are the least interesting places on the list (and except for HK, most expensive). Also, they are among the easiest places to access in China either on a future trip or future transit to somewhere else in Asia. My plan dumps them and gives you something else in return. You also can take a bit of time off HK and Xi'an.

Jun 12 arr HK 6 pm (HK)
Jun 13, 14 HK full days (HK)
Jun 15 HK part day. Cross border to Shenzhen at Lowu, overnight train K952 dep 17:26.
Jun 16 arr Guilin 06:45. Immediate transfer to Yangshuo rest of day (YS)
Jun 17-18 Yangshuo area full days (YS)
Jun 19 am by road (private vehicle) to Rice Terraces for rest of day and overnight (Dazhai)
Jun 20 midday by road (private vehicle) to Chengyang for Dong village/Wind-Rain Bridge. Dinner in area, then continue to Sanjiangxiang for overnight train K654 dep 19:45.
Jun 21 arr Chengdu East 13:23. Rest of day Chengdu (Ch)
Jun 22 Chengdu including Pandas. (Ch)
Jun 23 Chengdu area including dinner. Overnight train K6 dep 21:15.
Jun 24 arr Xi’an 12:15. Rest of day Xi’an (Xi)
Jun 25 Xi’an full day (Xi)
Jun 26 am bullet train G2002 dep Xi’an North 07:42 arr Luoyang Longmen 09:17. Morning Longmen Grottoes, afternoon Shaolin Si by private vehicle (LY) *
Jun 27 am by road (private vehicle) to Pingyao, with intermediate stop at Wang Family Mansion or Zhangbi Underground Castle if desired. Arr Pingyao 16:00-17:00 w/intermediate stop or mid-afternoon without. (PY)
Jun 28 Pingyao morning and lunch. Train 4626 dep 14:45 arr Taiyuan 16:22. Dinner near station then Bullet train G612 dep Taiyuan 18:06 arr Beijing Wst 20:52.
Jun 29, 30, Jul 1, 2 Beijing full days (BJ)
Jul 3 dep BJ.

* If wanting some mountain hiking, substitute on June 26 Huashan by bullet train from Xi'an, daytrip return with another hotel night in Xi'an. June 27 private vehicle Xi'an to Pingyao, similar time frame.

This plan completely gets rid of air travel, which in rainy season when you are going, is not only cost-effective but less likely to result in delays. Moving 8 people by air gets expensive. I have calculated the intercity transport cost per person for this trip and it is approximately US$350 per person. This assumes: soft sleepers for the Shenzhen-Guilin, Sanjiangxian-Chengdu, and Chengdu-Xi'an segments (with commissions for agent). 2nd class seats on bullet trains Xi'an-Luoyang and Taiyuan-Beijing, and hard seat on the short segment Pingyao-Taiyuan (seats all self-purchased in China). Also assumed are private van/bus cost of about US$570 total for the Yangshuo-Terraces, Terraces-Sanjiangxian, and Luoyang-Pingyao journeys....divided 8 ways. Hotel saved on 3 nights of sleeper train.