pg_plan_guarantee VS Hashids.java

For 4) I've heard there's this available https://github.com/DrPostgres/pg_plan_guarantee

Shameless plug(s). To address both the problems you described, I have developed the following Postgres extensions.

Index Adviser can be used to automatically analyze your workload, and it will suggest the indexes that might help your queries perform better.

Postgres Plan Guarantee (under development) helps you generate+freeze a plan, and ensure that the Query Optimizer will never pick a different plan for your query.

Postgres Index Advisor: https://github.com/DrPostgres/pg_adviser

Postgres Plan Guarantee: https://github.com/DrPostgres/pg_plan_guarantee

My preference: using traditional incremental numeric IDs and obfuscate them with Hashids (https://hashids.org) when exposed publicly

You basically want Hashids but sequential? Why not simple convert a base 10 (0-9) number to hex? (0-F)

I found hashids [1] to be a great compromise between integer ids in the database and copyable non-enumerable strings on the client.

[1] https://hashids.org/

I wonder how this might compare to just storing regular autoincrementing ints in the database, and converting to/from hashids (https://hashids.org/) at the edge. It eliminates the collision concern and stores more compactly at the cost of a tiny amount of encode/decode when processing requests. You’d want to push it down as close to the database layer as possible to avoid inadvertent int ID leaks; I added native hashids support to clickhouse but I’m not sure what other database support might entail.

Since hashids are not really hashes and are not secure at all this is not even achieved. Hashids can be easily decoded without the salt by a simple brute-force attack described by the authors of hashid themselves right on their website: https://hashids.org/

You don't necessarily need to use UUIDs. You could use something like Hashids to generate random strings from your sequential IDs in a reversible way, so that users can't predict what their values will be, but you can decode them as needed.

You should not use UUIDv4 as a primary key. You can use normal int values and then use hashids to make them safe for URL. UUIDv7 might be good to use as well once they are more widely supported as well.