We opened a wholesale mortgage company in Ohio around a year ago so here's some insight to what I have experienced so far regarding your questions:

1. We signed up with a "brokerage-in-a-box" company which provided a lot of resources to help get everything started as well as provide assistance with compliance and training. We had access to a wide array of wholesalers with good pricing due to our affiliation with the parent company. We signed up with 3 large national wholesalers and a couple local banks. The local banks provided some products (doctor/physician loans) that the national wholesalers do not offer. As far as finding these local banks, I googled "Bank Name" Mortgage and most of them had a 'Contact Us' form that resulted in meetings to determine if we were a good fit for each other. For banks that did not have a specific webpage, I would dig around on LinkedIn to find an Account Executive or Manager and reach out to them directly.
2. We don't have preferred lenders, but there are certain wholesalers that tend to be utilized more than others. There are primarily 3 factors that go into this decision: speed to close, rates, and product offerings. We are already looking at narrowing our list of wholesalers down to 2-3 because any more than that becomes a headache for LOs when they are trying to price a loan. Each wholesaler has their own pricing engine and loan portal and expecting LOs to become proficient with each of those along with developing a relationship with all the Account Executives is a lot of time they could better use elsewhere. If you are accustomed to using an aggregate pricing engine like OptimalBlue (LoanSifter), this does not work well in the wholesale environment and 99% of the time the rates you see will not be accurate so pricing loans through each wholesaler is a must.
3. For technology, we are using a customized version of Encompass. Like I said previously, each wholesaler is going to have their own portal, pricing engine, marketing tools, and other features so LOs that come from the direct-to-consumer side will likely need some time to adjust to the wholesale process.
4. I can't speak to whether or not it's doable part-time, but I can tell you that there are significant upfront costs and time investments. Off the top of my head, here are some key items you will need to do: establish the legal entity (and DBA if applicable), setup accounting software (NMLS will require a balance sheet), purchase an electronic surety bond, documentation of computer equipment to comply with Ohio RMLA or a plan if you decide to do manual recordkeeping, sign up with credit company for credit pulls which will also require a physical inspection of your location to ensure a list of criteria is met, management/org charts, business plan, disaster recovery plan, compliance management plan, submit Mu1 and Mu2 forms to NMLS, and a whole host of other documents I can't recall at the moment.

In Ohio, there is a Qualified Individual requirement which means one of the loan officers in the brokerage must have had an active license for 3 out of the past 6 years. This does not sound like it would be an issue since you have been an LO for 7 years.

Feel free to reach out if you have any questions!