After I had all the same issues of above, I did the following which solved all problems for me:
1. Made a backup with CCC
2. Removed my Samsung 850 EVO 1TB and did a Low Level Format on my Windows Server ( Low Level Format Tool)
3. Then formatted new HFS+
4. Replaced the SSD and booted with HIGH SIERRA DP1 Bootstick to do an absolut clean install.
5. After Configuration of iCloud, Apple ID and Networksettings updated Step by Step to DP 2.1. Enabling TRIM again.
6. Manually bring the files from Clone-backup back to the system
System was at about 18GB after this procedere and now during installations growing normally to 45GB. When I compare to my imac this seems to be correct. There it is at about 140GB..
There are no Backlight problems. Slow booting seems to be gone. System works very smooth without Fan overload.
The main problem I think is the conversion of the old used HFS+ to APFS and/or the enabled Trim.
When the work is over I will come back again and tell the last experiences belonging boot sequence time.
Only must get the machine MBP 13" late 2010 to active directory.