There are simpler ways around that.
1. Copy config.sys and autoexec.bat from D: to C:, and modify them as desired. Press Alt during boot to set C: as the default startup drive. If you remove cic100 and llras from autoexec.bat, you can have just over 410 KB free via Filer-Menu-Options-DOS. And if you remove 200 from the end, it won't automatically start Sytem Manager.
2. Press More-Menu-Application-Terminate All to totally close System Manager and have around 610 KB free. You don't have to worry about DOS=HIGH, since DOS is in ROM mapped to the UMA anyway.
3. Tweak DOS memory from Setup, or put a pipe character (shift-backslash) followed by a number immediately after an application's path in App Manager to give it that much DOS memory. I have DataPerfect with |384, for example.
Once you get used to the ways of handling DOS programs, you'll probably find that RAM is rarely an issue. Most programs from the 8086/80186 days aren't all that memory hungry.