martin VS pg_tileserv

This? https://github.com/maplibre/martin

Martin (https://github.com/maplibre/martin) or Tegola (https://github.com/go-spatial/tegola) as the vector tile server

Fine for a POC but there are much better libraries for serving MVT, for instance: https://github.com/CrunchyData/pg_tileserv

If it's small as in the couple-of-MB range you could convert to GeoJSON, serve it as a static file fed into a small web app like u/techmavengeospatial suggested that handles filtering by query, and update the file as new data comes in. Alternatively if you want something more robust you can feed it into PostgreSQL and serve with something like pg_tileserv (with equivalent filtering in the web client).

ArcGIS Online could be the right move as long as you set up the service correctly. You could also load the shapefile into a database like PostgreSQL and connect to the database via an API like Geoserver FastVector, or pg_tileserv. This would cause a lot more time, cost, and knowledge on your end though to host a database and api vs loading the data into the ESRI ecosystem.

One thing I will recommend -- because it seems like you are simple using flask to query the DB and render to the frontend (just from looking through the code) you **MAY** be able to use https://github.com/CrunchyData/pg_tileserv and cut out the need to even have flask as a the server side language. Curious what your thoughts would be because I am currently working on a few web apps and in the past i've mainly used geodjango and flask as server side but recently created an entire application with postgres, pg_tileserv, leaflet