Windows Vista!!! 😱
The ALT-Keypad sequence should remain viable for you. You can also create some snippets in Frescobaldi for those you use most freqently and assign a key-sequence for them, e.g., create a snippet for an ellipse … using Alt-0133 and assign, say, Ctrl-Alt-. as its key sequence. Frankly I'd think a cheat sheet with the Alt-keypad combo for your most used German characters is easier than creating an maintaining snippets.
Here's a big list of them with the upper and lower case letters:
à Alt+0224
á Alt+0225
â Alt+0226
ä Alt+0228
å Alt+0229
À Alt+0192
Á Alt+0193
 Alt+0194
à Alt+0195
æ Alt+0230
Æ Alt+0198
ç Alt+0231
Ç Alt+0199
ê Alt+0234
é Alt+0233
ë Alt+0235
è Alt+0232
Ê Alt+0202
Ë Alt+0203
É Alt+0201
È Alt+0200
ï Alt+0239
í Alt+0237
î Alt+0238
ì Alt+0236
Í Alt+0205
Ì Alt+0204
Î Alt+0206
Ï Alt+0207
ñ Alt+0241
Ñ Alt+0209
œ Alt+0156
Œ Alt+0140
ô Alt+0244
ö Alt+0246
ò Alt+0242
õ Alt+0245
ó Alt+0243
ø Alt+0248
Ó Alt+0211
Ô Alt+0212
Õ Alt+0213
Ø Alt+0216
Ö Alt+0214
Ò Alt+0210
š Alt+0154
Š Alt+0138
ú Alt+0250
ü Alt+0252
û Alt+0251
ù Alt+0249
Ù Alt+0217
Ú Alt+0218
Ü Alt+0220
Û Alt+0219
ÿ Alt+0255
Ÿ Alt+0159
ý Alt+0253
Ý Alt+0221
ž Alt+0158
Ž Alt+0142
ª Alt+0170
Þ Alt+0222
þ Alt+0254
ƒ Alt+0131
ß Alt+0223
µ Alt+0181
ð Alt+0240
Ð Alt+0208