I'd recommend against it, but then again, I'm building software products in this exact space with my launching customer being a 200-300M annual revenue logistics company. I don't think they could do it in-house. You don't give a lot of info on the exact niche you are in, but the username tells me that you're doing containers in European context, most likely shortsea or domestic, not deepsea. Me telling you the ISO6436 type code for your username is either LEG1 or LEGB depending on max stacking height should tell you I know my stuff :)
Your biggest pain point is most likely that you know your business very well, but you probably do not know enough about the business context of your partners. If you are running a small-ish container terminal for example, you really should know how the depot department of the major carriers are managing their empty stock so you can decide your stacking strategies in the yard. You should probably also have an understanding of benefits vs drawbacks of tower cranes vs gantry cranes found at bigger quays. That then influences what your TOS should be capable of. Same thing goes for intermodal transport planning, you really should know not only how detention/demurrage works, but also what tricks you can pull to optimize for it, and how shipping lines monitor this internally. If you are building stuff in-house, you're inevitably going to be limited in your understanding of the wider market. Short term that's a huge improvement (more tailored software, better flexibility), long term that might cause huge issues.
I can certainly understand the frustration with third-party products. Your description of 'they are understaffed and the platform is stagnant' could realistically apply to pretty much any software provider in this space. There is a huge opportunity for somebody to swoop in and build something, the bar for this sector is mostly something that doesn't straight up suck. And I'm going to be honest: we're trying to be that somebody.
If you'd like to talk, my contact details are in my profile. I'm happy to show you what we're building, and tell you what that is like behind the scenes. If you go ahead with your plan to in-house it, let's stay in contact, perhaps we can learn from eachother.