You can rollover your 401k into self-directed IRA, but not into HSA, which is Health Savings Account and not a retirement plan.
There are several reputable providers here on BiggerPockets who are active on the forum, just search the forum for related keywords and you will find out who they are. You are correct, reviews from their clients are very important, by reading those you can get a feel of what to expect if you were to become a client. Contact 2 or 3 to interview them personally then make your choice.
TD Ameritrade and Fidelity do not offer truly "self-directed" IRA, they might offer some control, but only within confines of their allowed investments, which is confined to the stock market and does not include alternative investments such as private lending.
For private lending, especially short term, you may find that you'll be better off with the "checkbook control". For that consider Checkbook IRA or if you are self-employed truly self-directed Solo 401k.
Many people on BP forums talk about many custodian it sounds you want a smaller more flexible company that can do both IRAs and HSAs. Look and talk until you are happy with the service they provide and have the attributes you want. Quick service, education, well designed website, access to account executives, etc. whatever you find important.
Check out Sunwest Trust. They are by far the cheapest self directed IRA custodian I found. I used to use Equity Trust, but the fees are ridiculous.
First, let me second what @Dmitriy Fomichenko (whose company is worth considering, by the way) said: stay away from banks and Wall Street institutions like Fidelity, TDAmeritrade and the like. Their people are ill-equipped to deal with real estate or private lending, and you will not have the professional support needed.
If you do not mind being overwhelmed, here is the (very long) BP list of SDIRA custodians.
The ones that you heard got bad reviews are actually pretty good. The anonymous internet has created lots of trolls. I have been a big fan of Quest for a long time. The founder still teaches classes across the country. He even lent me money way back in 2005 when I was just starting out. I have borrowed from a number of friends and qualified relatives and they have always been quick and pleasant to work with. Even the title companies have said that things have gone quite easily working with them.
I am sure there are many good companies out there you just have to make sure that yours does real estate. The stock and mutual fund companies are NOT what you are looking for.
Try IRA Services Trust Company. Also, New Direction IRA offers both types.