Basically all of the ISP’s at this point have some form of tuition reimbursement, so that’s one thing to consider. Getting a graduate certificate in really anything useful on CACI’s (or whoever) dime is a good place to start. There are all kinds of quick-hit online certification programs available through various universities, ranging from IT type things, computer forensics, white collar crime investigation, and more.
As others have said, Coursera and other online sources for free / almost free training are options too.
In terms of content / what to pursue, it really depends on what you want to do. If you want to stay in investigations, you can go into financial crime investigations and earn your CFE credential (time as an FI counts towards the work requirements to earn the credential).
If you want to get into cybersecurity, you can go Network+ / Security+ (or something like that).
If you want to go into security management, you can pursue ASIS’ CPP credential, and you’ll be exposed to all other areas of the security world, from IT security to physical security and so on.
If you’re already a Section Lead / Team Lead, you can pursue an MBA or something managerial and then try to get into Program Management at a defense contractor (I would not recommend an MBA unless it’s being almost entirely funded by someone else, they’re a dime a dozen for the most part).
The list goes on. I went into project management and I make double what I made as a Team Lead in terms of base salary alone. Literally my only relevant experience prior to making that jump was FI / Team Lead and some education.