The current stable release is 19.07.7, the next stable release will be 21.02.0 - 18.06.4 is obsolete and insecure, even the last maintenance release of this branch (18.06.8) is EOL and -by now- contains known security issues. You really must update, not soon - now('ish).
If you care about sticking to stable release, 19.07.7 would be the only option now - waiting for 21.02.0 would not be advisable (although there are 21.02 snapshots, if you're more adventurous or e.g. hnyman's builds).
18.06.x should upgrade upwards, although there are changes between 18.06.x and 19.07.x, the common parts in preinstalled packages should be more or less o.k. (important, don't restore an older config tarball, that circumvents upgrade migration steps which might be critical). It's roughly similar between 19.07.x and the openwrt-21.02 branch (again, there are changes, but the most critical things should be more or less covered by automatic migration scripts). That does not rules out potential issues in more complex addon packages. Be aware that these migration scripts do not cover downgrades, so these might break hard (just as applying a config tarball from an older release would on a newer version). With the pending switch from swconfig to the DSA switch framework, there is a major/ breaking change ahead for ipq806x after 21.02.x - this really needs more attention (in practice, starting with a fresh config from scratch).
Depending on how familiar you are with OpenWrt[0], how closely you're following it - and how complex[1] your configuration is, you may opt to try retaining your configuration over an upgrade. If you want to play it safe and simple, starting with a fresh config and selectively applying your necessary configuration by hand may be simpler.
Stable release upgrades are generally pushed because of significant security issue in (preinstalled-) core components and rarely done 'just for fun', so timely upgrades are strongly encouraged. Taking smaller steps inbetween might actually make this simpler, than doing larger upgrade more rarely. The more practice you get, the easier it becomes[0].
The r7800 has been a well supported for quite a while now, which is hard to brick and rather cooperative to the user, there's no particular need to be afraid about upgrades here. A powerful device which should work fine with OpenWrt for quite some time to come.
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[0] the more familiar you are, the more likely it becomes that you can handle rocky upgrades - making it easier to retain the configuration and selectively fix up the parts that need it. If you're not paying close attention, it's often easier to start from scratch and to only use your old configuration for reference - to apply your needed settings manually-
[1] the more complex your configuration gets, the more likely it becomes to encounter breaking changes - at some point it might be easier/ safer/ more reasonable to start over, rather than trying to debug issues and fix it up.
