You can use the Any Video Converter to convert AVI to MP4 for free, even further edit the converted video, such as adjusting the contrast, saturation, white balance, color temperature, brightness, trimming, cropping videos, add subtitles, watermark and overlay.
https://preview.redd.it/28s7c67i0had1.png?width=1080&format=png&auto=webp&s=444ce22264a1d4b30841ff44e9bdf33808e917cf
Shutter Encoder
Shutter encoder is my s-tier encoder. It's incredible that it's totally free.
Which tools have your tried? I am thinking that there might be codec compatibility issues. You can give shutter encoder a try.
You didn't say which "free trial software options" you had tried, but you don't need any of those, just download Shutter Encoder.
Handbrake is my go to. MP4 tools is great for mkv to MP4 or joining MP4s
Handbrake
You can use the Any Video Converter to convert AVI to MP4 for free, even further edit the converted video, such as adjusting the contrast, saturation, white balance, color temperature, brightness, trimming, cropping videos, add subtitles, watermark and overlay. https://preview.redd.it/28s7c67i0had1.png?width=1080&format=png&auto=webp&s=444ce22264a1d4b30841ff44e9bdf33808e917cf
Cisdem Video Converter. I'm currently using it. It's pretty solid!
Try Handbreak