Technology

Well to be frank, the fact that you're asking this shows you haven't really understood what makes something secure or insecure, or it isn't as important to you as you claim. If you want your stuff to be secure, your phone is the only "thing" that generates the 2nd factor. Especially things that are critical shouldn't have duplicate devices being able to also generate codes. If you do want to generate codes for less critical accounts somewhere else, you should register a 2nd TOTP generator with that service and use one each per other machine. That way, if something gets compromised, you can just revoke those devices preventing any damage without having to re-setup existing 2fa again for the devices that weren't compromised.

Now aegis is Android only, like you said. It also has no way of syncing with another instance (by design). It's local only, it can just do backups. Having it send the highly critical information anywhere kind of defeats the security-purpose of it being local only. It adds a whole communications protocol that has to be secured, and somehow you have to authenticate the other side and so on. This also probably doubles the complexity (or at least size of the codebase) for the project, which then makes audits harder et cetera. Aegis currently does one thing (generate TOTP codes), and does this very well and as secure as it can without compromises.

Now for an actual answer: Most password-managers can also generate TOTP codes, like KeePass or KeePassXC to name two open source ones. But it's their secondary purpose, with the primary obviously being storing the passwords. I'm not going to get into the implications of storing a TOTP code generator secret together with the password of the account it protects, let's just say there are some. Since the actual secrets are stored in a (secured) database, you can sync these between devices. Or you can just create multiple TOTP generators for a single service and keep them separate.

Or we circle back to something server based, like BitWarden, which is primarily a password manager but also does TOTP. It's a commercial, server based solution that is free for individuals. I'm not sure what the current limitations are for those accounts, like number of entries or just who you can share stuff with and so on. There is a open source implementation of their protocol called VaultWarden, where you can self-host the back end and not rely on the company securing their servers properly (and/or not being collateral damage in a breach of some kind). Again, combining password + TOTP-storage in the same service that is accessible online should be done with considerable thought to how it's secured, but you could use this to only store the 2fa aspect as well.