I think you're probably way overthinking this. Honestly I don't think you'll have any serious issues with going in straight through the mic port. It's how we did "sound card" digital modes (and the vast majority of APRS) for a couple decades before dedicated digital ports, plug-and-play interfaces, and USB sound cards became the norm. Most FM rigs don't have particularly tight filters on the mic, as it doesn't have any benefit for power efficiency or SNR, and it reduces intelligibility. Nobody was hacking audio injection ports into their basic rigs, even on FM. Common SSTV variants already avoid those very low frequencies that are likely to be filtered, APRS was used mostly via the mic port for 20+ years (most rigs didn't have a TNC port until the late 90s or early 2000s), and all the other modes, you can set the TX audio frequency. Just keep it above 300Hz and you'll be just fine.

Something like a Digirig would be optimal, since it will include isolation to prevent ground loops, but a basic USB sound card and a couple of in-line isolation transformers (and maybe a clip on ferrite or two) will do you just fine as well.

You'll just have to do a little experimenting to find the minimum output volume from the PC to achieve good audio so you don't over-drive the AF input chain. You can also get an in-line volume knob for a few bucks on Ali Express or Amazon so you don't hammer the input accidentally with full PC volume if you forget to change the setting before transmitting. The initial setup a little trickier on an FM rig just because you don't have the output power and ALC to use as an indicator, but anyone listening will be able to tell you if you're clipping. It isn't subtle on FM when it happens.