Selfhosted

I recently got upgraded to 10 Gbit fiber at home so I've been through researching this stuff.

With a 3G WAN, I'd go with a 2.5 Gbit LAN - 2.5G equipment is quite affordable now. The next step is 5G but that equipment is rare, and 10G starts getting expensive.

Do you know what router they're giving you? What LAN ports does it have? Does it even have a 2.5 or 10G LAN port or only 1G ports?

USB 2.5G adapters are available new for cheap and I've had good luck with them, even using one on a Synology NAS with an open source driver.

The wiring is probably fine as long as you don't have any very long runs. I'd keep it and only replace it if the links randomly drop down in speed to 1G.

2.5G switches also aren't too expensive. You can get one with only a few ports for the devices that can make use a lot of bandwidth (PC/NAS/Server) and plug your current switch into it for all the 1G devices like TVs, game consoles etc. The PiHole definitely doesn't need a fast connection.

I imagine you're talking about bandwidth and that your 300MB is actually 300Mbit/s and all your Gs are Gbit/s.
The fastest ADSL I've heard of is 24Mbit/s downstream, aka ADSL2+/G.992.5. You don't have ADSL.
I would guess that your "ADSL" is actually fiber and that your "Cable" is coaxial cable (same type that gives you cable tv).
If you wanna use more than 1Gbit/s your devices also need to support it. Even with WiFi 6 you will seldom reach 1Gbit/s so we're talking CAT6 cables and a motherboard that supports at least 2.5Gbit/s.

1 gig gpon is different to 1 gig direct access so it depends how the service is delivered.

But honestly changing from a 1Gb wan to 5gb or 10gb is a significant change in price especially if you need to use ppp.

I would stick with 1Gb.

If you want to start the most effective, upgrade your router or primary switch to 2.5G or 10G. Then at least there is a low likelihood of a bottleneck when your devices are communicating internally with each other and youll have overhead downstream. Then, if you have multiple switches, prioritize the highest bandwitch between them over upgrading your devices beyond 1gb nic's.

I use an opnsense router with 2.5g nic's, and then I have a 2.5g switch and a 1gb switch than are connected via a 10gb fiber link. (This is all enterprise ubiquity level stuff). But all my downstream devices and switches are 1gb snd I have no plans to upgrade intentionally. Internally, I won't see bottlenecks often since communication between the switches and modems is enough to support multiple devices spamming 1gb/s file transfers simultaneously (not that itll happen often lol)

So my WiFi access points, primary NAS, and my most used PC are all on 2.5gb connections since they could benefit. But everything else is on 1gb since the switch has way more ports and was way cheaper.

I'm not against buying 10g switches for future proofing, but they're still too costly for my needs, and its unlikely I'll wish I had 10g any time soon esp when it comes to internet. Even if I upgrade beyond 1gb fiber service, it'd be so thay multiple devices can fully saturate a 1gb NIC at the same time, not so one computer can speed test 3gb+.

Thay said, what I have is overkill, but i enjoy some homelab tinkering.