You might want to start with a simple scenario and step down to a C172 G1000 for now. It’s easy enough to step back up to the G3000 when time comes. There also seems to be a current issue with the G3000 following FMS, so that might be related to some of your troubles.
Pick two airports not too far from each other and accept the low-level IFR routing between them. Make sure that the Destination Airport has selectable Precision Approaches in the drop down menu in Flight Planner. (Navblue FMS database can be woefully incomplete - more on that below).
The good news is ATC does give you enough time to get to the published crossing altitude for the various fixes on the Approach. At about 5 miles to go, switch to APR or if you’re a stickler for procedure, wait until you pass the FAF. ILS should be very reliable, I’ve never had it not switch to LOC1, but, you’ll want to cross check the ILS frequency with a current approach plate. I have been caught off-guard where the sim auto-tunes the NAV frequency and it’s totally wrong (Navblue strikes again).
Climbs and descents - not sure what’s going on there, that needs more clarification. If it’s performance related, there are various free Modifications to change the performance envelopes and behaviors, but for simplification sake, I strongly suggest just sticking to the default performance model of the plane until you get more comfortable in this sim.
ILS for the most part is accurate. RNAV can and sometimes will underrate the glideslope, so cross check as soon as you can with the VASI/PAPI if visible, you may want to disconnect above MDA if you’re getting drawn down lower than calculated.
The G1000 in stock form works well enough. There will be complications when it tries to generate some SID/STAR routing using Navblue (in-the-box) FMS data. Weird turns, segments, etc.
Get used to that for a couple of flights. Feel free to turn up the complication by moving to a more intense airspace over time. You’ll quickly learn the quirks and limits of IFR in this sim as of today.
When you get sick of it, two things that will help immensely - 1.) Working Title Garmin Nav Unit modifications (easily installed/uninstalled and they’re free) - feel free to search for them in the board. 2.) Improved FMS data (by leaps and bounds) using a paid Navigraph subscription. Since they’re based on Jeppensen, you’ll find anything that you were missing in the default Navblue is restored in Navigraph.
I don’t represent either group, I am a mostly satisfied user/customer. I just flew four hops today, all IAPs using Working Title G1000 and Navigraph, and they all worked as expected.