Upgrade PostgreSQL 9.6.5 to 10.0 and Postgis 2.3.x to 2.4.x using Homebrew (macOS)

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After automatically updating Postgres to 10.0 via Homebrew, the pg_ctl start command didn't work.
The error was "The data directory was initialized by PostgreSQL version 9.6, which is not compatible with this version 10.0."

Database files have to be updated before starting the server, here are the steps that had to be followed:

# first stop force reload by launchctl
launchctl unload ~/Library/LaunchAgents/homebrew.mxcl.postgresql.plist

# need to have both 9.6.x and latest 10.0 installed, and keep 10.0 as default
brew unlink postgresql
brew install postgresql@9.6
brew unlink postgresql@9.6
brew link postgresql

# copy postgis 2.3.x to postgresql 9.6
ln -s /usr/local/Cellar/postgis/2.3.2/lib/postgresql/rtpostgis-2.3.so /usr/local/Cellar/postgresql@9.6/9.6.5/lib/
ln -s /usr/local/Cellar/postgis/2.3.2/lib/postgresql/postgis-2.3.so /usr/local/Cellar/postgresql@9.6/9.6.5/lib/
ln -s /usr/local/Cellar/postgis/2.3.2/lib/postgresql/postgis_topology-2.3.so /usr/local/Cellar/postgresql@9.6/9.6.5/lib/

# install postgis 2.4.x to postgresql 9.6
tar zxf ~/Downloads/postgis-2.4.1.tar.gz
cd postgis-2.4.1
./configure --with-pgconfig=/usr/local/Cellar/postgresql@9.6/9.6.5/bin/pg_config --with-xml2config=/usr/local/Cellar/libxml2/2.9.6/bin/xml2-config
make
make install

# move 9.6.x db files to another directory
mv /usr/local/var/postgres /usr/local/var/postgres96

# start postgresql 9.6
/usr/local/Cellar/postgresql@9.6/9.6.5/bin/pg_ctl start -D /usr/local/var/postgres96
# update postgis 2.3 to 2.4
psql your-database
=# alter extension postgis update;
# stop postgresql 9.6
/usr/local/Cellar/postgresql@9.6/9.6.5/bin/pg_ctl stop -D /usr/local/var/postgres96

# init new database using 10.0
initdb /usr/local/var/postgres -E utf8

# make timezone and timezonesets directories available for 9.6.x installation
mkdir /usr/local/share/postgresql96
cp -r /usr/local/share/postgresql/timezone /usr/local/share/postgresql96
cp -r /usr/local/share/postgresql/timezonesets /usr/local/share/postgresql96

# install postgis 2.3.x to postgresql 10
tar zxf ~/Downloads/postgis-2.3.4.tar.gz
cd postgis-2.3.4
./configure --with-pgconfig=/usr/local/Cellar/postgresql/10.0/bin/pg_config --with-xml2config=/usr/local/Cellar/libxml2/2.9.6/bin/xml2-config
make
make install

# finally the actual upgrade
# -b is the old binary dir, -B is the new binary dir
# -d is the old data dir, -D is the new data dir
pg_upgrade -b /usr/local/Cellar/postgresql@9.6/9.6.5/bin -B /usr/local/Cellar/postgresql/10.0/bin -d /usr/local/var/postgres96 -D /usr/local/var/postgres

# start 10.0 to check that upgrade works
pg_ctl start -D /usr/local/var/postgres

# cleanup if upgrade was successful
brew uninstall postgresql@9.6
rm -rf /usr/local/var/postgres96
rm -rf /usr/local/share/postgresql96

# enable auto load
launchctl load -w ~/Library/LaunchAgents/homebrew.mxcl.postgresql.plist

I'm trying to go from postgresql@10 to 11 with brew postgresql-update-database and it is failing with the error:
pg_dump: [archiver (db)] query failed: ERROR: could not access file "$libdir/postgis-2.5": No such file or directory

I've got postgis installed and updated (via brew install postgis) - and I'm not sure where to go next.

I've tried manually installing postgis-2.5.1 against postgresql-11.2 using the ./configure and make commands - but when running make it errors with:

ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
make[1]: *** [postgis-2.5.so] Error 1
make: *** [all] Error 1

I'm at a standstill ... any advice?